she got his number?!?!

My bf had been acting weird the past week so I asked wtf was wrong with him and he just kept saying nothing was wrong. But I just found his phone a few days ago and I didn't even have to unlock it before I saw a very inappropriate text from a random number. So I unlocked it and realized it was MY STEP MOTHERS NUMBER I confronted him and he just cried and said it had been going on for weeks and they had made out......then I spoke to her and she said they had a "connection" like I don't give a crap if you're soulmates that's messed up. What should I do????