Damn some people in this thread...
No, YANBU. It's your opinion, your feelings, you're entitled to them.
Call me sanctimonious, I don't care because it is perfect okay to decide not to associate with someone who displays thoughts and values that aren't similar to your own... that's how people find friends! There are some "deal breakers" and there are some things you're okay with them doing but wouldn't want to do yourself or around you.
FWIW I don't think the OP is upset about the cheating alone, it's the way this guy laughed about it, the "funny stories"... shows what sort of person he is. Given she's been cheated it's understandable she's have a "no happy cheaters" policy about her friends... so she doesn't need to have her P's friend as her friend.
There's very little point in the OP going to this thing with the friend... I think the night would suck because you REALLY don't want to go. You don't have to do ANYTHING to "make the bf happy" if it means it makes you unhappy.