Thanks for the replies guys and gals.
I'm pretty certain that the way it's going to go is probably going to solve the issue. While he's putting a lot of thought into the development of the plot it will be me that writes it and in fairness at present we've only used a small fraction of the ideas that could be claimed to be his alone. In the vast majority of our discussions, he's 'merely' a sounding board that shapes my own thought processes (while 'merely' is very valuable in that context).
He's doing it as a friend, and out of interest in all things literary - he's a poet himself - but, as with all these things money, even a relatively small amount, can change relationships.
The next step is to get it written and get interest from publishers (oh, only that then), by then we'll all have a clearer idea of his level of involvement and input to the 'finished' work.
As mentioned by Nik, this must be a common situation - in fact I wonder if any book is written without
any help from at least one person - so I could be worrying over nothing.
Of course, I literally
am worrying over nothing until the damn thing's written and published
Jon