If you guys aren't dating then I think you shouldn't think about it too much, since she's just a friend to you. If she was your girl friend then it would be a different scenario. And also, I would suggest not saying anything about this to her. If you accuse her of lying then I think this would hinder your friendship with her and she might not be the same to you. I say, let it go, don't make such a big issue about it. It might be that she wants some time alone with her boyfriend so let it be.