I would guess that the only real danger is from the oil paint and whatever is used to clean it up. The other stuff...grout, cement, and latex paint cleanup water, is pretty inert.
It might be an interesting problem/opportunity to do some research on bioremediation and set up a special
compost for such dumping if he insists on disposing of such stuff on-site. A pile of composting woodchips inoculated with mushrooms (see Paul Stamets' stuff) can break down just about anything.....