What you describe sounds suspect to me. Yes, of course, they can be just friends. When one is in a relationship it is generally not okay to spend the entire day with a friend of the opposite sex, socially. Wait, I just reread your post...did he stay at hers??? Obviously, people of the opposite sex work together all day long but it is when it is a social get together that it just doesn't feel right (to me) and it is definitely not okay if he's staying at hers!! I just don't think guys who consider themselves in a committed relationship would even think about walking another girl to work or spending an entire day with another girl much less spend the night at another girls flat!! So, in my opinion, yes he is stepping on the line. You should talk to him about your concern. If you are in a relationship with him your relationship comes first and he should respect your concern. If he makes excuses and continues to see her socially (even if she has a boyfriend) then he's not being respectful to you. Don't be a doormat and let him continue seeing her this way. Suggest that you go out as couples and get to know her and her boyfriend yourself but he shouldn't be seeing her socially on the level that he is. Sorry, I'm seeing red flags here.