he needs to check the fluid level in the differential. If it is really low it will cause a whine. If it has been run without gear oil for too long the gears may be really worn and also cause a whine and may need to be replaced.
Pull the check plug towards the top of the diff. If there are shavings on the plug (which most likely will be the case) then the rear end could be going out. Top it off with fluid and see if it solves it. If not, the gears are going out, so it time to have it replaced.