While I do agree that they do not know that the two did the horizontal mambo, we as the readers do know they did. The crew may only have a suspicion and Crisot played into it, but this is not a court and it doesn't matter if they actually did it or not. This is Wiley's wake and it is plausible enough that his endured presence would have caused a scene (or a bigger one as it already did). Best to leave and discuss the situation another time, if at all.
Not really a bad friend, he just has an attitude that puts himself before other people, not really a bad trait but it can lead to making decisions other people will have a problem with. Totally dick move with Melanie but it takes two.
Someone who puts themselves above their friends is a bad friend. Just, on a life-advice level, people who put themselves above you are toxic and we should all do everything we can to put them out of our lives, be they friends, colleagues, or family. I have a lot of experience in this area.