He’s cheating on his gf?!?!

So I started talking to this guy over snap, and honestly I liked him (as a person) and got along with him straight away. We kicked it off and talked very sexually and we even arranged to meet up and you know....

Then I go to his ig and find photos of him with this girl and he’s in a relationship...omg I have no idea what to do. I sent him a text saying “you have a gf??” But he hasn’t seen it

I feel so shit if I had of known I would have never talked to him or anything, but what should I do. Do I tell her? Part of me knows she has the right to know, but I dont want to be that person who ruins it...ugh I’m so stuck!! Please help and give opinions