He has a girlfriend

So I was at a friends house/ party and an old close guy friend of mine showed up. We use to talk until he broke my heart when I woke up to a text that says I'm dating blah blah blah.... So they've been going through a tough spot in their relationship and I was someone he trusted to talk to. All was good until he kissed me. It was like nothing before and it clearly meant something. He wants to get out of this relationship, and I want him to because it's a toxic one. Not just to better myself. Should I tell her what happened? What do I do?