My bestfriend fucked my fiancée

I have been friends with my bestfriend since the 7th grade! Over the summer she moved in with my fiancée and I, and our son. A couple of weeks ago I found out they've been fucking the whole time she's lived here. Since then I have left, and have gotten a job. But now she's constantly messaging me telling me about how great their relationship is and even goes as far as sending me videos of them fucking. I'm seeing red! I don't want to go to jail for hurting her but she's pushing my last button! she also says soon my ex fiancée won't need to see our son because she's trying to get pregnant. He hasn't tried to contact our son at all.