theres a monk in my current dnd AL game who apparently gets temp hp when he knocks an enemy to 0 hp (also, he thinks they stack, which I should inform him is incorrect). After looking at every monk/subclass feature in any supplement (because this seemed to good to be true, and also ‘cause i’m Jealous because i’m supposed to be the tank), I think he may be lying. I don’t think he has multiclassed. Could somebody tell me if this is possible?

You should talk to the DM about that, or ask the player where that features comes from. It might be a magic item.

Without a magic item, I think the only way they could get temp hp from dropping a creature to 0 hp would be from being multiclassed into a Warlock of the Fiend.  I might be wrong in that, do Long Death monks have a similar feature?

That's an ability that Monks who take Way of the Long Death get, and you're correct it does not stack.

Ok, cause that seemed op, but I guess it’s just that the temp hp doesn’t stack

The temp HP is WIS + level, they get the ability at level 3. As others have said, temporary hit points do not stack. Also, the monk has to be the one that reduced the enemy to zero. It can't just have died nearby them.

Edit: fixed temp HP math

Quote from Houligan >>

The temp HP is WIS + 1/2 level, they get the ability at level 3. As others have said, temporary hit points do not stack. Also, the monk has to be the one that reduced the enemy to zero. It can't just have died nearby them.

 Unless a new version has been released since the SCAG, I believe it's actually Wis + level ? They have to be the one to deal the killing blow though.

yup, I just can't read today.