Well, it has to be fixed. Not safe to trust for a rollover as it sits, I would get several quotes for the ROPS parts (at least the lower half), then show them to him and ask him if he is going to pay cash or file with his insurance co. If your Sister is taking his side and it looks like it will be getting ugly, I would drop it. I am all for having him pay for what he broke, but there is a point where a good relationship with ones family is worth more than a $500 ROPS. If it looks like it will be getting to that point, I would put him on my "not allowed to even touch any of my tools" list and let the matter sit until the next time he wants to borrow something.
As the the actual work, I would install it myself if the shop wants $175/hr. That much will buy a set of 1/2" impact sockets, a breaker bar and a set of wrenches. It appears that the replacement of the lower part entails pulling 8 bolts from the tractor and 2 from the ROPS hinge. You and a helper shouldn't have any problem swapping it over.
Aaron Z