Specifics matter. For this feat, the attack is granted as a free action. In this specific case, your homebrew monster attacks, does damage, and stuns. The free action does not state that it interrupts the attack, and therefore resolves after the attack. Because the character is stunned, the resolution fails due to not being able to perform the action.

Immediate action states:

If an effect has a trigger but is neither an immediate action nor an opportunity action, assume that it behaves like an immediate reaction, waiting for its trigger to completely resolve. However, ignore this guideline when the effect has to interrupt its trigger to function.

Therefore, as the feat has to wait for its trigger to resolve, and it's not explicitly an immediate interrupt, and stunned explicitly prohibits taking actions, the character stands there and looks stupid.

Specifically, unlike "no actions", "free actions" are things in themselves. They belong to the class "Action." There are no specific rules allowing free actions during stunned, therefore they are forbidden by the wording of stunned:

The creature can’t take actions.

This kind of "don't bother this turn" is why the stunned condition produces such copious amounts of no fun at all and should never ever be part of an at-will attack.

Talk it through

This turns out to be the answer for a lot of issues like this. D&D is a social game and the only way to resolve many of the issues that come up between players is by talking about it. In an MMO, it's easy to just drop the group, kick a player, or whatever...a D&D party has a lot more investment in it.

The best solution to this sort of problem is to sit down with the other player and talk it over. If you aren't comfortable with this, go talk to your DM. They should be able to help you. Here are a few pointers...

Talk in private

You don't want to call the other player out in front of the rest of the group, that's a quick way to make someone defensive. Instead, catch them before or after the game when you can talk privately.

Focus on the game, not the person

You don't want to rip into another player over this...again, that just makes them defensive. Instead, focus on the game and the characters, and how their abilities can interact. Talk about your respective roles and responsibilities, and how you can best work together to enjoy the game.

If they are more experienced in the system (or at RPGs in general) than you, consider approaching it as asking for advice

To this particular case...

"Hey, I noticed that our character's abilities are kind of interfering with each other. Your Sorcerer keeps knocking enemies all over the place, and I'm trying to round up enemies to keep them focused on my Paladin, so they don't go kill the Druid. I mean, the opportunity attacks I'm getting are nice, but I'm going to be a lot more effective if I can keep enemies close to me. Do you have any suggestions for how we can fix it so our characters compliment each other more?"

Talk to your DM

This one doesn't apply as much in your particular case...but for broader applicability...consider a sneaky character in a party full of rushdown players ("who needs scouts, let's just charge in!"). This is a case where the entire game is clashing with a particular character's skillset, rather than a single character messing up another's ability to operate.

In this case, the best option is to talk to the DM about helping develop situations where your character can shine. Again, the goal here is to have fun...most experienced DMs will look at their playerbase and try to engineer situations where all of them get their chance in the spotlight...but sometimes a DM needs a nudge.

Solve problems with out-of-game solutions whenever possible

One thing you absolutely do NOT want to do is start having your character act out against their character. In an MMO, it's pretty common for a Tank who gets mad at another character to stop protecting them. "Whoops, rogue pulled again, I'll taunt it after he's dead."

Don't do this. All it results in is mad players, hurt feelings, and arguments that can tear the entire game apart. Especially don't do this if you have not talked to the other player about what's going on. They (probably) don't know they are upsetting you. They might even think they are being helpful or making you more effective.

Talk to people to solve problems...don't take it out on them in-game.

Regarding this Particular issue...

You referred to World of Warcraft several times, so I'll reference that game as well. Essentially what's happening here is that you have a character who keeps pulling mobs off the tank. Imagine a character (like an Elemental Shaman) using a knockback ability to punt mobs off the Main Tank. Sure, it does damage and momentarily halts that mob's ability to do damage, but they are hampering the abilities of the tank and it's probably going to get the Healer killed. In a typical WoW group, such a player would get kicked from the party, or the tank and healer would drop out of the PuG. In D&D, kicking a player or dropping out is a much bigger deal, so the better option is to work through the problem in a social manner.

Talk about it, figure out what's happening, and how you can all work together to fix it. After all, the ultimate goal of D&D is to have fun!

This particular case is more of an issue of the Controller not realizing the best way that his powers can be used alongside yours than him having a 'different playstyle.' Sure, knocking enemies away from you gives you an Opportunity Attack. But if he instead used his 'move them around' powers to pile enemies up on top of your Defender, you can use all of your awesome defensive powers to wreak absolute havoc on your mutual foes.

In 4E, at least, the optimal use of a Controller is to lock down a few enemies to keep them out of the fight so you can deal with them later and/or help keep the enemies piled on the Defender. If an enemy wanders off to go attack someone (like the Leader), then the Controller is best served to knock that enemy back over to the Defender or lock them down so they can't hurt anyone.

If the Defender has most of the enemies rounded up, the Controller does best taking a few of those enemies, and tying them up somewhere away from the rest of the party so they aren't dealing any damage to anyone. The same way that, in a WoW Raid, you may CC a few of the enemies to keep the pressure off the tank, so the tank doesn't have to fight quite as many enemies at once.