Caught older boyfriend cheating. Best revenge?

I'm (F21). My boyfriend (M56) went on tinder behind my back two days before valentines day. I logged in. He texted this 18 year old that said she needs groceries and asked him for 150$. He said "Sure, if you give me back a favor in return. Where are you located?". She asked him to transfer it now and he said "I'll give it to you when we meet". He told her that hes at the VIP room in a certain casino every weekend and to come see him. She said shes a broke student and doesn't drive. He asked for her phone number and said "You won't regret it. I'm a real sugar daddy" along with "You're cute" "I like and want you" "Hope you are real babe" "I'm an investment banker" "Damn you sound cute". Hes claiming to be 36 on his profile but he is actually 56. He is also using his son's name instead of his own name.

We live together 80% of the time and a lot of my stuff is at his place.

Hes away on a business trip right now. I was supposed to go but I canceled last minute because I couldn't lie to my parents. He said he wanted to bring someone else with him to help him drive because he was "tired". (Its a 3 hour drive). I begged him not to and he finally gave me his word that he wouldn't.

If this is the only instance I've found out about, I'm sure there is more. I've caught him using Chaturbate too, even after he promised me he wouldn't do it again.

Best revenge? How can I expose him without him knowing I logged in? Should I text her? What do I say?