This is pretty difficult to call. The actual situation could be anywhere from long-standing good friends with nothing going on, used to seeing each other for days at a time + jealous, controlling boyfriend; to perfectly reasonable boyfriend + girlfriend who has an inappropriately close/romantic relationship with another male, or is even cheating.
You need to talk with your girlfriend, not the internet. Ask her what is going on and why she lied, listen to her thoughts and feelings on the subject (and explain yours). Stop thinking just about what's "normal" - if you trust that she does actually view this guy platonically, will you have a problem with it?* If you don't trust her.. well you obviously need to think about that and what it means.
*Although, she does seem very involved with his guy. I could understand you being a bit jealous (for the attention), even if it was a platonic female friend.