Duct tape (if available) can be used to secure the strips of rubber in the tire, keeping them separate from the tube, preventing them from shifting around, and adding a little more protection.

I've ridden in really thorny country and have a health respect for Slime. The bulk containers are pretty affordable, and it probably is the best option. Your friend can put tape over the holes, let the air out, add the slime, and pump it back up. The tape only needs to hold (at low pressure) long enough to distribute the slime. For small punctures the slime will seal the tube. He'll need to patch any large holes first).

If Slime is unavailable or cost prohibitive (for the number of bikes in the picture it shouldn't be) a reasonable equivalent can be made with latex mold builder, but these mixes frequently use ammonia and anti-freeze, both of which I would try to avoid. Vegetable based propylene glycerol can be used, but again there is an availability issue.
Do a search for "ghetto tubeless" and it should turn up the relevant info.

I used to ride in an area so covered with goat's head thorns that I couldn't ride a hundred yards without getting two flats. With slime and thick tubes I managed to ride 5 to 6 days a week for two years with only one flat. The one time I did get a flat I removed about two dozen thorns, pumped the tire up, spun it, waited til it stopped hissing, aired it up the rest of the way and finished riding to work. For all the issues slime has it definitely has it's place.