Unless I'm misunderstanding you, for dialog it is no more complicated than what you described, and it is not difficult to execute in FCP. I'm not sure what your sound guy is having a problem with. Again, dialog is usually recorded mono. BTW, if your sound guy is really mixing audio and doing sound design, he should really be doing it in an audio program like Protools, Nuendo, Cubase SX or Logic and not in FCP. FCP or any NLE is great for the indie film guy and much can be done with them, but if he's a "sound guy" he should be using sound tools and not video tools. It would make his life, and I imagine yours, much easier.
Perhaps we need a section here on audio post.... I'm recommending a few books.
Matt,
Sorry if I was unclear. He's editing the film. He has no more experience in sound design than I, but he is a kick-ass film editor. Maybe next go-round we can have a dedicated sound editor.