Biggest tire he can fit is 285/55/20 he will need a 0 or -6 offset I wouldn’t go wider than 9”. He will have rubbing on the mud flaps if he has them take them off, fixed. He will also have rubbing when at full lock hitting a hard bump. A good alignment will limit the amount of rubbing. Does he know the side wall is going to be really small and weaker when off road? 20” wheels on these trucks aren’t ideal. Biggest wheel on these trucks that still looks ok and has function is 18” IMO.