He has a girlfriend

I’ve been sleeping with this guy for just over 3 months I think, and I’ve just found out he’s got a girlfriend. He’s in the Navy and he’s based in my town and his girlfriend lives a good 2 hours away so I guess that’s why I never found out. I really want to tell her because damn the girl deserves to know but it’d have so much backlash on me via my friendship group so I don’t know what to do😭😭 I wanted to ask a friend to tell her for me secretly but I don’t wanna come across as a bad person!! What do I do??

Update:

I’m gonna try and find someone to message her and tell her for me, the thought of doing it myself is making me so anxious so I’m gonna ask around and find a girl who’s willing to help me out. Is that a bad idea??