What would you do in this situation?

I was talking to a close friend of my best friend and the close friend accidentally slipped and told me that my best friend has cheated on her current boyfriend in the past. I trust that my best friend tells me everything so I have no idea why she's...

hiding this because it's not really that big of a deal compared to other things she ever told me. But I'm not hurt about it because the close friend assured me that my best friend didn't tell her. She found out on her own and asked about it and so my best friend confessed to her since she's already busted.

I really want to ask my best friend if it's true and why didn't she tell me, or anyone about it. But I'm not and I really don't plan to because it doesn't matter to me if I'm not close with someone. If they tell me not to talk about the conversation I had with them, I won't do it. The close friend didn't tell me not to, but it's just common sense to me that I shouldn't because then, they might have problems and I do not wish for it to be my fault. What would you do?

I would talk to the close-friend about it and see what she thinks and if you guys should tell your friend about it or ask her what was going on.

oh by the way, it's such a coincident that she just happen to bring up feeling guilty about thinking of someone else while she's doing things with her current and she felt guilty about it. I asked her.. well, DID you do anything like cheat on him? she told me NO.