TBH being older and wiser, i would take him to the bar, buy him a drink and ask him what the issue is, then explain nice and polite, staying calm what your situation with this girl is. don't let him break in with "but...." or "yeah but" say your peace, ask him if he understands what you've said to him then pick up your beer and walk away. Pat him on the back before you leave and say "good man" so he thinks your a maniac.