Transmission does not have first, reverse or overdrive

If it is a 4L60E it could be a few things, most common for loss of those gears only = sun shell failure. Lo-reverse clutch pack worn out or blocked apply circuit to the piston.
Best option is a rebuild with a beast sun shell and heavy duty clutches. Next would be to replace with a remanufactured or used transmission depending on what condition the truck is in.

If it is a 4L80E the low and reverse band anchor pin is a likely issue. They can break or the case fails.

For both the losses can also be electrical and should show up on a good scan tool. It could be failed solenoids or bad wiring if that is the case. A good visual of the wiring and transmission connector and scan should show if those are the problem.

Water in them can cause problems. If he has a decent scan tool he can probably check to see if it is electrically related.