He may just try doing what my Father in law did. He has an old revolver and he figured the grips were too small for his massive hands. So he took a 2x4 and made his own grips. His grips are about 1-2" longer then the normal grips or frame for the gun. It's actually rather comical, but after he smoothed, sanded, cut some lines for gripping and stained it it's not TOO bad.
So that could work for if he wants a revolver. If he wants a semi auto get a normal big semi auto and then get an extended magazine for it and make a spacer.
Mainly joking with the stuff above, but I would agree with a double stack 1911 or Beretta 92/96. Those are the biggest normal guns I know of. Other then that I would say go the biggest shop he can find and ask for "The biggest handgun ya got."