This one is tricky. My wife was in a similar situation and said it was strictly platonic and to not worry about him. I was jealous and insecure and felt like he was trying to date her but she would deny it and tell me there was nothing there and not to be jealous. They would text and go out dancing/clubbing with a group of friends. Well one day he breaks up with his girlfriend and told my wife that he had feelings for her since the beginning, which freaked her out and she ended up blocking him on everything. She said she was just oblivious to it and was excited to have a friend with similar interests. So like I said it could be a tricky situation, but she could be oblivious to his advances (although the talk about dating a crush while in a relationship is kind of a red flag, right?)