@Boone42,
If your relationship is just 4 months old, and you are already seeing a therapist because you feel she is dishonest. It also seems you don't trust her enough to meet with a male friend of hers without hooking up with him, despite being involved with you. It doesn't sound like the basis for a good relationship to me, but ok.
Two scenarios:
a) She is going to meet him with the intention of hooking up with him, or establishing a relationship with him. If that is the case, then you won't be able to stop her regardless: all that not trusting her and ging to therapy is going to do is make up her mind for her she would be better of with him than with you.
But I doubt that's the case, because she is going to meet him together with you, right? If she really wanted to hook up with him, she would meet him alone: much less hassle that way, after all. Besides, this guy lives in another country apparently, and that brings its own problems to a potential relationship as well.
b) She is going to meet with an old friend, and that's it. If he does live in another country, it's probably not all that easy for her to meet with him often, so it makes sense she'll try to meet him if she is able to. She's taking you along as well, which I interpret as a good sign, see above.
I guess it just comes down to trust. If you can't trust your partner to not try and cheat on you, then what sort of future do you see for yourself with that person?'So go along with her, and just see what happens: if she is truly as callow as to try and hook up with this guy while you are there, then she isn't worth starting a relationship with in the first place.