IMO, you CAN make a plastic ball back up with negative sideweight, but it would take so much that the ball would not be legal. I have used an old 12 lb urethane ball as a "trial ball" for several ladies considering going to a fingertip grip, so the ball now has 3 or 4 sets of holes to the negative side of the last grip I drilled in it. The trial was to see if they could be comfortable throwing a properly fit fingertip grip and the ball obviously was not used in competition. The last lady I did that for normally throws a small hook with her conventional ball, but the trial ball had a definite backup. When we drilled the same grip in her Emerald Vibe it had a definite hook. I haven't weighed it, but I would guess it has about 8 oz negative side weight.
For your friend, I would suggest sanding a plastic ball to 4000 Abralon, then polishing with Bean's Secret Sauce - burn it in good. When he lines up for the 10 pin have him align his body so his swing to the target is in the same plane as for his strike ball and see what happens. It may take a while for him to get comfortable with his shoulders being open to the foul line. -- JohnP