My opinion (and only that) from what you have written:
No, he hasn't overstepped the mark. The question was fair, and was probably more focussed on his need to get round writer's block than on you in particular. It might have been an opening gambit in developing a friendship/relationship. Even so, that's not wrong, providing a polite negative reply is taken as such.
"What does one do?"
Politely decline. A simple "No" may invite a "why?" so something a little more full might be a better conversation closer if that's what you wish.
"Have you asked xxx? They're more into fantasy." might be a deflection, if passing the buck, but "I'm not comfortable with one-to-one collaboration." would be honest, and should serve.