Usually Bentonite. It'll make putting out the fire hard as hell unless they've got something besides water, but it's relatively harmless. It's used by the truck-loads in municipal water treatment.
-But-, there's lots of fraccing substances who are also very water-reactive, and dangerous as hell. After looking up these guys' services offered, though, they seem mostly into work-overs and environmental work, so Bentonite is the best possibility.