Father in law cheating

So this is a little complicated for me as I’ve never had a good relationship with my mother in law until recently.

So a year ago my father in law was caught cheating on my mother in law, which made it the second or third time he’s been caught. She stayed again but they started to try counselling.

Anyways, a week ago my fiancé and I bought a bus and my FIL came along for the drive. I sat in the middle back seat since my kids car seats were on both sides of me. Since my FIL is 50+ years old he keeps his font size at the highest. He was texting a woman on WhatsApp which I thought was weird. (I was only looking at his texts to begin with because I initially seen him mention, rudely, that my son who is 8 months old was fussy because he hadn’t stopped for lunch). So I continued watching the text conversations and noticed he was sending multiple texts at a time to this woman whereas sending one word answers to his wife.

THEN he started sending kiss faces to this woman and was telling her how much he misses her. Also making plans to call her because “he was going to be away from his wife for the day”

I don’t know if I should tell my MIL since this will be the third or fourth time this has happened. It been really hard on me bottling it up especially he belittles me constantly throughout the day.

Yesterday he told me I was allowed to work on the bus (my new home) because I’m a woman and my job is to watch the kids and that he will be the one working on the bus with my fiancé. So when he left his phone at my house I took a screenshot of her number since he has deleted his messages and call logs, just in case I needed it.

What should I do?