It's definitely a legal grey area. I'm a born skeptic, so I'm inclined to think that most of those "pharmacy club" type sites are just out to scam the desperate and the addicted. However, my guess is that yes, one could purchase darn near anything online--there is at least one Mexican pharmacy online that sells Provigil, an Rx alertness drug, online, as I found out while researching the drug.
The trouble would likely be getting it past Customs--although it IS legal to bring in up to 3 months' worth of prescription meds into the U.S., you have to actually bring it in yourself. Obviously, some of these foreign pharmacies are able to get SOME past Customs SOME of the time, but it's a gamble, and the DEA doesn't look kindly on importing controlled substances without a valid, written prescription from a U.S. licenced physician.
So, I'd be very surprised if your relative was actually able to have Percocet mailed to him legitimately. I'd also advise him to keep his mouth shut and not advertise his method of obtaining the drug. Why does he have to go the 'grey' route? If his current physician won't prescribe needed pain meds, then that physician needs to explain, in detail, why he won't do so, and provide other alternatives.