the reason I brought up the sexuality was because of the op's friend having said that perhaps he had done it because he needed something op couldn't give him. 
Fact is that often when someone cheats with someone of the same sex people sympathise with them in a "it's so sad because he/she must have been so confused/hurting/ and is so brave to finally be out..." etc, whereas if they cheat with another man/woman they are just cheats.
In the op's case she knew her p was bisexual so that side of things isn't an issue. but it is naïve to think that for everyone it wouldn't be or that everyone should be comfortable with the notion of having a sexual relationship with someone who has slept with someone who is the same sex as they are. I don't care what anyone's sexual preferences are and if they want to have a relationship with men or women or both, as long as all are consenting. But I wouldn't personally want a sexual relationship with someone who had had sex with another man, that is my sexual preference - to be with only heterosexual men... in the same way as I know that many homosexuals have issue with people who are bisexual... etc. It's personal choice...
If I found out my dp had cheated with another man it would be a double betrayal because A, he had cheated and B he had lied about his sexuality, but if that happened I have little doubt that he would still get the sympathy.
My friend's dh decided that he wanted to become a woman, at a time when they were going through the process of adopting a child. Obviously he had always had these feelings but the fact is she was oblivious until he dropped the bombshell. He had gone through the façade of marriage and wanting to become parents/then applying to go through the adoption process and then dropped it on her at the most inopportune moment. And he was the one who got all the sympathy "oh how tortured he must have been/how difficult it must have been, surely you have to understand what it was like for him," while meanwhile he left a devastated woman behind who has never got over the fact theat her entire marriage was a sham and that not only that but she couldn't divorce him but due to the fact he changed identities the marriage actually had to be annulled. 
Op I would drop your friend like a hot brick and keep doing what you're doing.