Is he using a pocket knife to get the last bit of tire over the rim or something? If tubes are getting ruined by trying to put a tire on, he must not be taking care to keep the tube well inside, and it's getting pinched between the bead and rim. If rim tape is getting ruined by putting a tire on, then ... ? I can't imagine what would do this except for violent use of inappropriate tools.

Some tires are just tighter than others. I'm surprised that a MTB tire gives this much trouble, because usually wider is easier.

Make sure, when getting the last bit of bead over the rim (whether the first bead or the 2nd) that the rest of the bead is well down in the depressed center channel ALL the way around the rim, that should give the necessary clearance to get the last bit of the bead over the rim with HANDS only. One trick I've seen to make sure the bead is well-centered (and this was for Schwalbe tires) is to tie string around the tire in a few places to make sure the beads are pulled together and centered. Or zipties.

If that still doesn't work, look into getting a bead jack, or give up, take the wheel and tire to a bike shop, and offer them like $5 to put it on. (good luck fixing a flat on the trail though...)