Cheating! Would you rather

Ok so I went on a date with a guy who I thought was single. Later that week, we had sex. I found out he has a gf of 3 years, she's studying abroad right now, but her insta shows they're still very much together and she obviously loves him. Some of my social circle knows her and they agree she's a super sweet girl who has no clue her "perfect " bf is doing this. Also I witnessed him hooking up with another girl a few weeks back. I feel used and mortified and angry and super sad for her. Should I anonymously tell her what this guy's been up too? Even if she doesn't believe me, I need to get this off my chest.