rod and reel got ripped off!

   And there is another fine example of why they shouldn't outlaw handguns
   Just kidding... BUT......... I feel your pain but there is little you can do that won't escalate the problem.

i would just chalk it as a loss

chances are he'll just yank on the line and cut it off when he can't pull any longer, then leave the rest (out of sight, out of mind).  that much fishing line will foul up the seal around the prop shaft.  when that happens, water will get inside and ruin the entire lower unit.  he's looking at $3K worth of repairs if that happens.  by the summer, he will be staring at his checkbook and cursing your name.  he's not a fisherman and he won't know what fishing line will do to a prop shaft seal.