Hey Mom I Hope You’re Doing Well

Mom had a baby when she was 17, and she was a great mother. She was very caring and loving, always listened to me, and had a good sense of humor, as if she was my friend. We were about to reconcile until I found out she was talking to a guy she met in college. No one knows about that guy. I have no idea who he is but I know he's a threat to her and my family. I'm very angry with her and I don't know what to do.