Urethane made for rafts has huge elongation properties that allow it to flex and stay bonded while it flexes. From what I understand Linex is formulated to be sprayed on stiff surfaces that don't require a whole lot of flex or elongation. If the Linex has low elongation properties it will pop loose from the boat when you roll it or the boat flexes. Check your buddies spec sheet and see what the elongation is. System 6 is 200% The other bonding issue you may have is with the material. I would get a piece of hypalon, prep it, and have him spray that to see how it bonds/ flexes before committing the boat.
The other issue you may have is that unless your buddy can spray it smooth, linex will be rough and will hang up on rocks easier instead of sliding off like the urethane coatings on rafts.
Good luck and post up the results, I have always been interested in how it would turn out.