Lol I love this 'guilty until proven...well, guilty all the way' attitude
. I'm gonna guess that at least half of you have no experience on this particular subject.
Anyway. Okay there's nothing wrong with a healthy dose of scepticism, trust me on that it's the best way to get through life. However, I don't see all this jumping to conclusions. Why is she necessarily a potential cheat or why must she be losing interest in the OP just because she does things with this guy?
I used to live with a girl a couple of years back at uni who had a long distance relationship with her boyfriend. You see where I'm coming from with this? Basically it was a very similar situation. Incidentally I was into this girl I lived with.
Guess what, we went places togther, usually just us two. We went the cinema, went for a nice walk through parks (yeah I was an AFC at the time), went for meals togther etc etc. Obviously I didn't need to text her, but we spoke a hell of a lot.
And you know what, nothing happened. Ever. Absolutely nothing. Even if we went on a night out together nothing ever happened. If anything she'd come back drunk and go phone her BF.
So believe it or not, no matter how good friends they are, even if he's into her, it still doesn't necessarily mean anything will happen.
It's all a question of well you know your GF.