I know it sounds like I'm not any fun, but
I Wouldn't Allow It
Getting close to the sleeping koi killer is easy: The koi avenger makes an opposed skill check (the koi avenger's Stealth skill versus the koi killer's Perception skill). The koi killer takes a -10 penalty to his Perception skill check because he's sleeping. If the koi killer wins, he awakens. If the koi avenger wins, the koi killer's asleep.
But then nothing I can find short of mind control--be it with the skill Sleight of Hand or otherwise--permits the koi avenger to make the koi killer--awake, asleep, or unconscious1--take something from him, be it something beneficial (e.g. ioun stones), malicious (e.g. pariapt of foul rotting), neither (e.g. girdle of opposite gender), or some combination (e.g. bag of devouring). The game just doesn't allow forced-equipping. The koi avenger could drop the girdle on the koi killer from a great height for possible damage. The koi avenger could lie to the koi killer about the girdle's provenance and give him it... or sell him it. The koi avenger could even stash it among the koi killer's unattended goods.
But, according to the rules, the koi avenger can't strap the girdle onto the koi killer without the koi killer's permission.
A DM can house rule this, of course. It could be this simple: The DM allows the koi avenger to take a full-round action to make an opposed skill check (the koi avenger's Sleight of Hand skill versus the koi killer's Perception skill). The koi killer takes a -10 penalty to his Perception skill check because he's sleeping. If the koi killer wins, he notices the attempt and the koi avenger provokes an attack of opportunity from the koi killer. If the koi avenger wins, the koi killer doesn't notice the item among his possessions. If the koi avenger beats the koi killer's Perception check result by X or more, it's also as though the koi avenger used the item normally.
But even if X is outrageously high, that's an incredibly dangerous precedent. That's partially because of what's going on in the question ("How can I force foes to wear the clothes I want them to wear?" to which the answer should be, "Use magic to make them"), but also because of one ancient legacy spell, the 8th-level Sor/Wiz spell trap the soul, which reads, "As soon as the subject picks up or accepts the trigger object [for the spell trap the soul], its life force is automatically transferred to the gem without the benefit of spell resistance or a save." Emphasis mine. (Also possibly a concern is the host of spells involving scraps of paper that can then be deposited on foes or by foes (e.g. explosive runes, illusory script).)
You think adventurers are paranoid now, wait until they're surrounded by admirers in the town square yet must burn their clothes afterward--without touching them, maybe while in them--for fear of accidentally touching a triggering object for the spell trap the soul that anyone could've placed in their pockets. Expect careful inventories of every single item every single morning--by scent so as to avoid accidental contact. Expect lots of wasted time. Expect naked adventurers.
Cursed Items and Vengeance
Cursed items are funniest when users suffer because they are careless, greedy, reckless, or whatever. A cocky adventurer is supposed to find the girdle, foolishly assume it's a girdle of giant strength eminently suitable for his massive form, bully the party into letting him have it, flex a little, strap it on, and--bam!--he's a lady. A good laugh is had by all, and everything's back to normal after the adventurer's the target of an effect like the spell remove curse et al. The adventurer will know better (or not) next time to run some tests on a found magic item or have a magic item identified before he uses it. There's a point to that.
Changing the sleeping koi killer's gender is not revenge. That's a prank, what with it being "generally lighthearted, reversible and non-permanent," and aiming "to make the victim feel foolish or victimised." In fact, I'd even argue it "involve[s] cruelty verging on bullying" as it's being "performed without appropriate finesse." If the koi killer really wants revenge he needs to make it appropriate, do it with impunity, and make sure the koi killer knows that the koi avenger did it and why the koi avenger did it.
I'd advise the potential koi avenger to reconsider and instead just haul off and sock the koi killer, saying, "That's for my fish!" Unless, of course, that fish needs avenging, in which case such avenging should be done correctly and with style.
- Okay, there's this: "A character can carefully administer a potion to an unconscious creature as a full-round action, trickling the liquid down the creature's throat. Likewise, it takes a full-round action to apply an oil to an unconscious creature." But that's more like casting a spell on someone than making them accept an item.
Tasting isn't quaffing.
Chances are he didn't gargle the potion to get a feel for what was in it, he got a bit on the tip of his finger and then rubbed it on his tongue. He would be able to tell from just a bit of a taste that no this is not a potion ow why is my tongue burning.
If he explicitly states that he sucks the potions down like a vacuum cleaner then yes, he would probably die. The flesh eating acid would likely eat through the lining of his stomach, and then melt a hole through his buttocks. He'd die in a very very painful fashion. It sounds like he deserves no less, just based on his actions it'd be a great way for him to die.
You said he did take a quaff so that pretty much sealed his fate. Poisons, acids, and potions aren't to be fooled around with, and his inexperience will teach him a gruesomely painful lesson. The lining of his esophagus will melt away, his teeth will rot, some will drip down into his lungs and begin burning through his alveoli, will drop down into his stomach, burn through its lining, where it will singe a hole through his intestines, pouring his stomach acid into his bloodstream until it passes through his buttock and falls onto the ground. Any bones the acid come in contact with will be eaten away, and this entire time he will have no shortage of pain to deal with as he breathes his last.
Best Answer
Roleplaying games are rarely perfectly balanced even for their intended use of PvE. PvP balance is nonexistant. It all comes down to who has the most access to and the most incluence on the DM. That's no fun, that's metagaming to the extreme.
Take AD&D 2. All it needs for your PC killer is to say "Next night when all of them sleep during my watch, I slit all their throats". That's it. By RAW, that's all it needs to kill a party of sleeping, defenseless people. Not even a dice roll is necessary. There's no challenge in it. At all.
What I have seen done is that the DM let the PC kill the others, then gave him his reward and told the story how he was never happy with his reward and always paranoid and got killed by the BBEG's henchmen after the BBEG won because nobody opposed him. That's a pretty clear message from the DM to the players to not do this, or there will be no fun.
Personally, I'm a grown-up and I don't need such a time-intensive way of bringing simple messages across. Just sit down and tell people what you want and don't want from the game. Not wanting a game where the team plots and kills each other is perfectly fine. You said it was your mistake it started (1), so own up to it and correct the mistake. Sit down with the scheming player and tell him you are sorry and it was your fault he misunderstood your intentions. Nevertheless, you would like to change how this plays out. You can offer him a way out (like in turn betraying the BBEG and gaining a reward that way) or you can hope you have a mature group where people are motivated by having fun together without being rewarded for every step they take.
If your players want to have some PvP fun once in a while, suggest a boardgame. Boardgames are balanced around all players trying to get each other. They can be fun, too. And they don't ruin another persons time-investment.
(1) It takes a special kind of player to actually take your offer. Anybody at the table should know that killing another player character unfairly (and there is no other way in RPGs) will diminish another players fun. Not another characters fun, another players fun. Make sure you really want such a player in your group. Such a behavior is anti-social and you are trying to have fun with a social activity.