If he can find a factory ROPS for his tractor, that's fine. If one doesn't exist, wouldn't a well built, home made one be better than nothing at all?
There is no system out there that is 100 percent for every situation. My dozer has a factory built Full ROPS on it, and it's not rated to support the tractor in a full roll over. Just a side roll or to offer some level of protection from debri falling or hitting it. Same with my Full ROPS on my backhoe. Both of these are full sized, commercial machines.
And for those who say roll overs happen so he should have a ROPS, I will point out that more people are ran over and killed crossing the street than in a roll over. It happens, and even with a certified factory system in place, there is still no gurantee that it will save your life every time.
Eddie