Should I message my ex’s new gf for cheating?

So we’re married and was going through a rough patch. I was staying with my parents but we were still seeing each other on the weekends, hanging out, going out with our daughter, and sleeping together and he was coming over to see my daughter during the week. But then I find out he’s had a gf that lives an hour away for the last 3 months!! And he kept it from me while I was thinking we were going to fix our problems. I even asked multiple times if he was looking for someone else and he denied it. WE ARENT EVEN DIVORCED. Oh and he’s living in a house that MY PARENTS gave to us and took this new girl over there. I’m so disgusted. Even after he told me about her, he tried to sleep with me. Multiple times. I have proof of it so should I let her know? She’s posting them all over Facebook and saying how “blessed and thankful she is to have such an amazing man”. Boy does he have her fooled.