Of all the faults Perry has, informing Wade that Ian fucks his girlfriend is pretty mild.
You know I hate Perry as much as anyone but telling Wade if Ian and Cindy are fucking would not be a fault at all. At the very least he should confront Ian about it. Wade is a friend of both Ian and Perry, Ian fucking his girlfriend behind his back is awful behaviour and totally against the bro code.
Now the Emma issue is a bit more complicated. Emma is not Perry's girlfriend and although he has (reluctantly) expressed an interest on her, he hasn't made any move. So I would say is morally grey for Ian to fuck Emma (closer to bad if he did after telling Perry he would help him), not exactly cool but it's a sudden attraction, shit happens and Emma wants Ian and not Perry, what are you gonna do?, you can talk it out like adults. But the thing that makes Ian a really bad friend is not telling Perry at all about it, encourage him to pursue Emma and even let the poor idiot confess to Emma just an hour or two after Ian fucked her, knowing that he's gonna be subjected to ridicule and rejection. Perry has a right to be totally mad about it, at the very least Ian should have stopped him at the last moment from confessing his feelings and save him some dignity.
And if Ian does both things, he's the worst friend ever and it's on a clearly unforgivable situation. His friendship could only continue if Perry is the biggest cuck ever and has a huge aversion to conflict, which he doesn't, the idiot can't keep his mouth shut to save his life.