we have a couple of group projects and presentations coming up that we are working on together and I don't want my marks to suffer. (i'm on track for a 1st class degree) I also don't want to lose her as a friend.

Is it at all feasible for you to speak with your professors & be assigned to another group? A woman who can casually cheat on her own husband & risk destroying her family is NOT someone you want as a friend or confidante. Even if you can't get out of working with her, you need to begin phasing her out of your life. Do not confide your own personal issues to her anymore.

FWIW, I was in the same position many years ago (also while I was in school). I had no physical proof to offer, but felt a moral obligation to tell the injured party for the same reasons other people are mentioning (e.g. concerned the person was unknowingly exposed to STIs, etc.) I felt it was the right thing to do, and was told the same by other friends (who also knew what was going on).

Long story short, I got completely shit on. I was blamed for being a backstabber and ostracized from the friend group. The very people who told me it was the right thing to do (but didn't have the balls to do it themselves) were now giving me the silent treatment. It made group projects all but impossible. They barely spoke to me; to top it off, they dumped the entire workload on me to complete. Eventually, the injured party found out everything, but it's not like people ever apologized for treating me like the harbinger of doom. I was just another casualty. And all for "doing the right thing".

So it's up to you. Sometimes the right thing to do is not the smart thing to do. Personally, I'd ensure you had a new study group lined up before dropping this bomb, because it WILL tank your friendships & your reputation, and make your academic life hellish. That said, I've never regretted telling the truth - my conscience is clear - but I wish I'd planned for the fallout better than I did.