<human>: So my friend, we'll call him A, has a new girlfriend (C). I don't know her personally, however, I do know that she was one of several other women that my friend's (different friend, B) ex slept with during their relationship. This girl then used her cheating escapades to get attention by talking about it and saying that because B had forgiven the douche in the beginning of the relationship for some infidelity (we're talking before they were official) that it justified C's decision to knowingly sleep with B's boyfriend a year later. 

So the meat of the problem is this. Do I warn my friend A? I'm thinking of sending an anonymous note so he can't trace it to me and he can do what he wants with the info. I'm afraid if I go to A directly he won't believe me because I used to be into him and I feel he might think I'm trying to brake them up. I'm just worried for A's feelings and potential health. But at the same time I don't know C personally and maybe she changed her ways? 

And maybe I should just but out, but I feel like if I were A I would want to know. 

Side note: although I once did like A I know for a fact that he is not interested so I'm no longer pursuing that. So I'm consciously trying not to mess with his personal life. Which is why I haven't just talked to him yet.

Can you condense the text into three sentences or less?
<bot>: Friend is dating a known cheater, I don't know if I should interfere and tell him that she's cheated in the past.