<human>: I really feel I need to talk about this with somebody but am scared to tell my close friends - your objective thoughts would be helpful!

I've been dating a girl for the past 4 months or so, and It's been great. We've held off making anything official because of a painful past relationship of hers - I'm ok taking it slow. We've become extremely close and spend a lot of time together, and we feel what we have is special and more than just a fling. 

However I'm struggling to cope with a very difficult element - since we've been dating she's been open about the fact she has feelings for a close guy friend of hers [m21]. Before we were a thing they slept together once but she called anything off with him, saying she doesn't want to be with him and that it was a mistake. She's been totally frank with him that she has stronger feelings for someone else (me) now and wants to pursue that. 

Obviously this is tough for me, especially when they spend time together.
I could deal with it just about, until more recently when he's been much more intense with expressing his feelings for her. I don't think he cares about the fact she's chosen to be with me, I'm pretty sure he wants to break us up and is being quite manipulative - and I'm worried his intentions might be working. 

Last night a text popped up from him on her phone during sex and she had to stop saying just seeing his name made her upset and totally not interested. Clearly this was awful for me, I had to leave. 

I don't know what we should do. I'm really falling for her and the thought of giving what we have up is devastating... but how do we continue with this messy situation? Part of me wants to talk to him, and basically plead for him to back off - but is that not my place? I don't even know the guy personally.

Thanks for reading and your thoughts would be appreciated!

Can you summarize the given text in a few sentences?
<bot>: The girl I'm seeing has feelings for her friend and it's causing us both pain - how do we deal with the situation? I really don't want to leave her.