snooping

So i was snooping through my fiancé's phone the other day (i know i shouldnt have, but i did) and i found a txt message from this girl from instagram that i had told him to stop talking to and she had the guts to tell me to leave him alone. I know a girl wont say something like that unless they have something going on, i confronted him about it and he didn't even say sorry about it or ill block her or anything like that i have 2 kids with this man and idk what to do any advice ladies please