Always try to cheat with me on his girlfriends

I’ve known this guy for years and we were friends for a while. We started hooking up but his friends told me not to take it serious due to him being a cheater. I said okay.

Recently he adds me on social media and I was shocked but accepted it. We exchanged numbers and started to text.

I seen on social media a pic that says “happy 1 month” & I was like what?
I told my colleague and she suggested I ask “are you seeing anyone?” So I did and he stopped texting and the next day he says “no, are you?” I said “no.”

So we hang out a few times and everything and I see a picture of him with her. I was like “ok…”

We continue to talk but I kept getting a gut feeling to back off.

I see now that he’s been in a relationship with this girl for a year… (it’s been a year since he added me and started to talk)

I have screenshots of him admitting he was single and all that. Would you wanna know if your boyfriend cheated a year ago? Should I have messaged the girl right away? I didn’t wanna look “crazy”

I feel bad for her because I know his ways and I rejected him because of him being a cheater.

(GUYS THIS ALL HAPPENED A YEAR A GOOOO, I did drop him and I just seen the girl post that they have been dating for a year! Meanwhile he was talking to me. I’m asking is it even worth showing her he was cheating from a year ago)