If the bow he's wanting to convert has a version that comes in single cam, you can call the factory and ask what size cam and what length string and cables you need to make a certain draw length... as long as you're going back with a cam made by that company that is.

If he's going to use a cam from a different manufacturer, then he's way off into making a frankenbow. In that case, there are too many unknowns. Only thing to do is try it and experiment with different string and cable lengths. Frankly, I don't think it's worth the effort.