Am I the only one who read this and though you were massively overreacting?

He didn't hide it, he told you that he wanted to meet a person that he considers a friend.

You got angry and he ended up staying out because going home to you was almost certainly going to be a continuation of this fight. I'm not saying he handled things well, but it sounds like you don't really respect him very much, he's doing a PHD, and he has no release.

Maybe he shouldn't have this kind of relationship with a woman, but it sounds like you aren't making it easy for him to be at home. You said he had innocent chats with her, but you're upset that he didn't tell you about them? Ok he lied, but do you chat with him? Do you guys have fun? Or do you just push him from one obligation to the next and then keep score about who does more in your relationship?

To me, I would be on your side here if it weren't for the fact that he clearly ran it by you before going to meet her and even after checking into his facebook messages you didn't find anything incriminating.

You should consider couples therapy, and really have open communication.