What to do about my cheating mother?

The last 6 months or so my mom has seemed extremely suspicious. My curiosity has gotten the better of me, but I have seen text messages from unknown numbers and facebook messages with the guys name saying things such as "Missin U baby xxoxo" "Hope I can come up to see you see for a long one, and then some quickies..love u (my moms name)" and all about how much he loves her, and my mom returns the messages equally. It really hurts me and it makes me want to hate her, but I really want to tell my dad because he is the nicest person and doesn't deserve to be cheated on. I'm 15 so it's basically out of my control, any suggestions?