May be he did it in purpose, may be not. If he is with another girl, and you like another guy, then I suggest to ignore it and move on. I know people are curious about what is happening around them, but some curiosity is harmful to our feelings and thoughts. If he was a friend, and no longer so, why do you keep him in your "Friends" list? I am assuming that he cannot tag you if he is not a "friend". It is just a suggestion if he bothers you.