Are the rear wheel bearings on a Pontiac Grand Prix the same as a Pontiac Bonneville?

first of all ..if you ever want to move the GP it wont roll... and wheel bearings are cheap and you will need the wheel races that come with the new/old bearings

Installing used bearings would be a waste of time and effort.

Not a good solution. He should put his money into new bearings. The GP ones probably are not interchangeable anyway.

I would think they likely are the same, but you want to be sure before going to a bunch of work for nothing. Any auto parts store should be able to tell you by simply looking up the part numbers to see if they are the same. Rear wheel bearings are pretty simple to change. They are a one piece assembly that simply bolts on. There are no inner races to worry about as was mentioned above. If the used bearings have over 100,000 miles on them they won't have much life left in them so think about that before swapping them. Ask him to put the old bearings onto your car so you can still move it around if you need to.