I've bumped in to stuff with my old Kubota and with my recently acquired New Holland. I have scratched the ROPS, but never more than that.
If this was a $50 repair, I would chalk it up to expierience and not loan it again.
But, as I guessed from seeing the pictures, I was right at about $800-1000.
I would seriously be talking with him, and handing him a bill. That is just totally wrong to do that much damage, and not fix it. And not tell you the whole story.
I'd be checking all the ROPS attach points real careful too, to make sure there are no cracks or anything.
Once repairs are done, I would make it clear to him that he does not even get to sit on that tractor(or anything else he might need to borrow) ever again. He can rent or buy!