<human>: A couple months ago I swiped right on a guy on Tinder. We were both just looking for a regular hook up / f*ck buddy. We met up once and he was a bit apprehensive, although very confident in messages.

He told me that he was in the Army (I live near a military college) and as an officer he didn't use Facebook in case his subordinates saw. As such he had a fake profile just for Tinder with a couple pics. He also said we should communicate using the Confide app because there phones are searched and he didn't want his superiors finding out he was f*cking around. 

We met up and had sex a few times. He'd come to my place, we'd do it and he'd leave. No problem. Once when he was having a shower his phone went off and I picked it up (bad, I know). From the name in the phone and the content of the messages it looks like the sender and he are together. I took the number down and confronted him when he got out the shower, telling him we were done. He left telling me I was crazy.

I feel awful for this girl. I don't want to reveal my name or number to her (so I don't get in the middle of it all) but I could send her a Facebook message. The thing is, I have no evidence. The Facebook and Tinder profiles are gone and there are no conversation histories or logs. It would be a message telling her "Hi, your boyfriend had sex with me". The only way I can think is to tell her things about him that I know, but it could all be lies to me.

What should I do? We had unprotected sex once (I've since been tested and in the clear) too, but he could be doing this with other girls.

Can you give me the gist of the text in a nutshell?
<bot>: Had sex with a guy from Tinder a few times who I found has a girlfriend. Have her phone number but no proof of anything between us. (How) Do I tell her?