I (F23) found out my FWB had a relationship a lot longer than he let on

[Elara]

So around February of last year I (F23) started hooking up with a friend (M21) from high school. We would sleepovers or just see each other in general pretty frequently. He had established that he did not want a relationship which was fine with me. In May of 2024, he finally ended our FWB situation because he had gotten a girlfriend. I was a little bummed but I respected the situation. A few months go by without hearing from him until he writes me in August asking if I wanted to come over, which I declined due to him having a gf & our previous "relationship." He wrote me again in September asking me to come over to hook up again, which was again declined. He has since not written me but we still are friends on social media. The other day I see a post of him and his girlfriend celebrating their ONE YEAR ANNIVERSARY. Which means for the first 5-ish months of their relationship (and another 2 attempts later on) he had been cheating on her with me. I feel awful for not knowing and wondering if I should tell her? I no longer have any messages from Feb-May between him & I, only having his request from September. I'm not really sure if this is enough to send to her or if she'd even believe me. I have not been in this situation before and would really appreciate some advice.

TLDR:

Dude I was hooking up with was in a relationship the whole time we were sleeping together and I just found out. I had assumed he only started dating his girlfriend when he ended things with me. Please give me advice on if/how to tell his gf?