Have talked to this girl for a long time now, but I don't know

I need help, so I have talked with a girl since July, and it got serious around 2 months ago. I talked otp with her today and she told me in a deep Convo, that she basicly f*cked a friend of mine in August. I'm not that good friends with him but he's my friend. I dident know about this I'm not tigheter with her rn but we are almost tghter. I don't know what I should do I really like her but u don't know if I can keep on when she litreally fucked someone I know while we talked, (we were not serious at all when it happend that's the thing)