Hmmm. This is a really good question. This was not one of the options you listed, but I would probably approach the guy before he left the restaraunt. I would walk over to his table, speak to him, and INTRODUCE MYSELF to his date. Now as far as your options go. I would tell my friend also. I would do as others have stated and ask leading questions. But, I would definitely tell her. I would not want him flaunting some hoochie around town while my girl is at home thinking she is the only woman in his life. I can't have that. (Of course I would tell my friend how incredibly ugly this other girl was....even if she was a supermodel
)