6' rough cut mower home made or not is awful big for that 15 pto hp tractor imho there may be more wrong than you think?
My 38 pto hp 3810D pulls the 5' howse RC I have and it loads it pretty decently in the thick stuff.
As far as proving it are you meaning whether he damaged it?
If it was ok when he borrowed it (think I am getting that right? ) then he is responsible since it was in his care, custody and control thats how the law where I am reads anyway on liability.
edit: and furthermore my policy is you break it you fix it! I rarely borrow tools but when I do I find some way to bring it back in better shape than I got it in.