As a professional photographer I was a bit of a hard case. In this situation my thought would be to say thanks but no thanks. But you may have another read on the situation. It seems to me that you are asking for a world of trouble with this other dude poking his nose into things. And, yes, you need to learn how to fire clients. Another person might try to sit down with the client and outline why having another photog along is nothing but a disaster in the making. Maybe he will get it. If not, then you can fire him. Just a thought.