Ok… I understand that question title seems like it's a bit harmless, and I'm sure others have asked similar questions but I'm not entirely sure if this fits the same bill, or if it even a problem.
So: to the matter at hand. One of our players is cheating – but to make it interesting, it's only half the time. Honestly, I can't really pick any pattern, I only get a glimpse every other roll*.
Some examples might be attack rolls. I've seen them substitute lower rolls for higher ones, but it seems like only half the time. Nothing fantastic, like crits all the time, just enough to pass the roll. However, they do accept crit fails as well.
During our last couple of sessions, this behaviour did seem a bit more aggressive, and his character (which he has become very attached to) did come to the brink of death (2 hp) by the end of the quest.
I'm really not sure what's going on exactly to tell you the truth, but if I had to take a guess, they're cheating to keep their character alive, and doing a good job of it to keep from getting noticed. I'm really not sure how to deal with the situation.
So, to update this: the player is definitely cheating. I have been keeping tabs on his rolls, and a large majority of their rolls are 15-17, about 70%. Additionally, the majority of these "streaks" occur when the PC is getting low on health – using them to pass checks that will stop the PC taking any more damage*.
I have also got a couple of ideas of the cause of this as well. Firstly; the player is very attached to the PC. They have issues with separation anxiety, and the idea of losing the character really does upset the player. On the other hand, the DM is fairly brutal. I have been playing with this DM for a few years now, and he really does like pushing players to the edge – forcing them to use all their resources. It makes sense, but it does create a certain paranoia.
I don't really want to bring this up in front of the whole group, and openly accuse the player of cheating, especially when it's for such a minor reason, however, I don't think speaking to the DM about it is really going to sort things out either, since "backing off" for one player makes it unfair on the rest of us.
Additionally, we have experienced one PC death in the past few months – my PC was actually killed, due to a series of unfortunate rolls. While the event was a little upsetting, we were more upset by the rolls, than anything (no one managed to get higher than a 4 on the dice).
This also makes the issue a fair bit more delicate, since the player may take it as an act of "revenge"; they are comfortable cheating so that their PC can stay alive, but are not willing to do so for other players. I just want to clarify that this is not the case – my concern is that I do not want this issue to develop any further. The player is seemingly cheating to save their own skin, which is a minor issue, but I don't want this to evolve into anything more, that could potentially ruin the fun for everyone else.
How can I address this issue with the Player and the DM, so that no one feels the need to cheat?
*I would like to clarify how I know the cheating is occurring. Firstly, I have caught him red handed on multiple occasions (though obviously never said anything), calling incorrect rolls: they might call a 17, when on the dice, it's actually a 4. Additionally, I have also been noting down the rolls, and comparing them with the PC's health, and these "high streaks" always occur when the PC drops down below 10 HP.
Best Answer
If you're worried that people are cheating on their dice rolls, you need to have a norm that people roll their dice in public, in full view of the table. Most tables have this rule already: you roll your die into the center of the table and everyone can see the result.
It's probably a bad idea to accuse the guy of cheating to his face, but it seems like you could have a quick private word with your GM, say that you're pretty sure the guy is cheating, and ask him to remind people to roll in public.
This is leaving aside the broader question of whether it might be okay, occasionally, to cheat deliberately, if it will keep a character alive. Character death is pretty unpleasant, and many groups will bend the rules to avoid it. (The 3.5 DMG explicitly describes some circumstances under which the DM might cheat to avoid a TPK.) But players aren't supposed to lie about dice rolls -- that's a violation of your group's social contract -- so in that sense this is something that should stop happening.