Most Helpful Guy
Honestly since you two are already at that point in the discussion you might as well tell her the truth. Make sure to come off as sincere and genuine about it. You might save her from future heartbreak if they do get married but still that can damage the trust they have with each other. If you hadn't been in touch with her I would've said no because he "could" have changed and that its his/her relationship and he has already moved on thus you shouldn't bring in your past relationship into theirs BUT I do not condemn cheating so it's really 50/50. But it all comes down to you; it's your choice and up to you if you want to let her know or not.