Tell the provider you are hiring to pretend she is just some old friend you met at the mall or something and had drinks with one night...and she let you know how attracted to you she was. I don't condone lying in relationships, but you did not ask for my opinion on your relationship, so all I would suggest is not letting him know what you do. Just make it as though you have always wanted to do a 3 way and this is his bday present from you to him. He need not know either of you is in the biz.