Confronting a deal breaker....

So my spouse has been chatting for several years with this woman from his past. She was his probation officer when he was a minor and they had a very inappropriate relationship then. Fast forward 30 years they reconnect via FB. I recently told my him that contact with her is a deal breaker for me. He cut communication.... Until last week. He doesn't know that I am aware of their 4 hour text session or that I read those messages. My question... How do I confront him?? As I had told him she is a deal breaker for me, I feel I have to take action.