You probrably already checked this but first of all -
are the tires completely glued on all around the rims
does he have the same compound tires on all four corners,
-if not are the front and rear the same left and right
-and are the rear softer than the front
If these are ok and the rest of your set up looks ok, then changing the diff grease or oil should fix the problem.
put something very light in the rear so that you can spin the rear wheels freely by hand. ( in oil about 5000 )
put something medium to heavy in the front ( in oil 30,000 to 100,000 ) Im not sure about weights of grease.
This should fix the problem but the car will tend to understeer or push in corners so work your way back by reducing the front diff oil weight .
as long as the rear diff is realy loose and the front is very tight the car will be stable - then you will have to work your way back to get a good balance for corner speed, and more drive out of corners
A good start is 5,000 rear and 30,000 oil in front for tight corners
and 10,000 rear 50,000 front for larger open track.
I'm not sure of similar grease weights - maybe someone else could help