They should not put in tubes. If you have tubes, just use water.
Part of the reason you use rimguard or window washer fluid or anti freeze is to keep the rims from rusting. (the other part is to keep it from freezing)
If you do a search for "loaded" or "loaded tires" you'll find lots of info (and opinions) on the subject.
I'll probably be admonished for doing it, but I'm running straight water. It's not cold enough to freeze here and keeping the rim completely submerged should slow the corrosion process. I figure if I get a flat, water won't hurt anything, it's cheap to replace and easy to work with. If I have to replace the rims in 20 years, so be it. You can buy an adapter at Tractor Supply for 10$.