I think your friend needs to first determine what he does best, or most efficiently, then ask for input on that area. Wheels vs. engine parts, etc.
Myself, I'd like to see wheels available in 1/25 & 1/24 scale 18" to 20", but you'd need to survey to see which sizes make sense businesswise.
i love the suspension parts idea (think R-B Motion), but it has to be cost effective.
Good luck.