Friend & morals troubles....

I have 2 friends, that after high school I haven't talked to much, but still care about them. They are both female and 1 is very much a lesbian. She is fantastic! She has had the worst time the last 2 years, her brother (foster brother) committed suicide a while back and a month or so ago her foster dad passed away. Her foster mom is also having health issues and her older sisters are in and out of jail. But even with all of that it is no excuse for what she has done. My other friend has a boyfriend and my lesbian friend KNOWS that they are together have been for about 2 years. On and off of course. Well my lesbian friend had sex with my other friends boyfriend.... Idk if it was just once or more than once.. I am so disappointed in her. I have always hated him because he cheats on her all the time and it's totally my friends fault for taking him back every time. She doesn't know about this though and they are all friends.... Idk what to do because I feel bad for not telling my friend, but we don't talk much anymore and I don't like sticking my nose in things.... But I feel so awful about the whole situation... Any advice??

***Added**

Her having sex with him also caught me off gaurd. But I promise she is a lesbian. I have no idea what made her do that with him because she doesn't like him that way.