Most Helpful Guy
Send him a message in Facebook and tell him that you know what's going on and that, if he keeps it up, you tell dad. Also, learn to take screen snapshots so you can get a picture of any incriminating evidence.
Meanwhile, when you are alone with mom, in the car let's say, simply be cold about it: "Mom why are you fooling aroubd with Mister X on Facebook?" It doesn't matter that you violated mom's privacy-she's still guilty. If mom punishes you, you punish mom by going to dad.
Basically, you need to be proactive toward nipping this in the bud and spare dad if you can, but if you can't, you tell dad.
Most Helpful Girl
Call her out. Tell her you know exactly what she's been doing and ask her direct questions about it. This guy is an outlet for her and she is using him to make herself feel better, to feel connected, to feel more confident (my gut). Tell her you know and take it from there.