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?

[Sony12]

Stay out of it. He was honest with you about what he was looking for from you. You don't know what was going on between them. She very well could have been hooking up with other guys as well. Just let it be. They seem happy together now so don't attempt to interfere in their relationship when he was honest with you about what you were to him the whole time.

[ExpatInItaly]

I would also advise you stay out of it. 

You don't know the terms of their relationship, if they were both dating  or sleeping with others before becoming exclusive, if they have some sort of open arrangement, or if she would freak out on you and come after you instead of him. 

It just isn't worth getting involved. 

[smackie9]

No you don't say anything to her, but you do confront him about it, then end your friendship. He's a user and a loser.