<human>: I had picked up her phone since it fell on the floor while she was asleep, and she had several messages from people. One name I did not recognize so I just checked to see what it was about. (she checks my phone and looks through my stuff all the time, but I don't mind). Then I see this long conversation about stuff involving her best friend, planning a threesome, and the whole ordeal. 

I'm trying to figure out why she would do this. We've been together for 11 months. We get a long great. Our sex life is pretty good. And we talk about marriage and having kids together all the time. Then I find out this threesome nonsense. I'm pretty open to a lot of sexual stuff, but if I'm not going to be involved in this kind of thing with her, I'm not sure what to think.

I'm pretty sure her best friend is the root of the problem. She already cheats on her boyfriend, and won't dump him because she's too scared of the fallout. I've told my girlfriend to put distance between them, but she can't seem to do it, doesn't want to do it, or won't do it. I don't like the best friend much after I found out she was cheating, and now I like her even less since she seems to be bringing my girlfriend into her shenanigans. 

Do I confront her about what I found? Or try to trust that she will chicken out and realize what she's planning on doing?

Can you summarize the given text in a few sentences?
<bot>: gf is trying to plan a threesome with her best friend and random guy behind my back, what should I do?