If it just stopped running while he was driving, it may have a piston similar to the one above now. Pull the injectors, blow out the water. After you have blown out the water, replace the injectors.
Now, you should be doing this everytime you get in the truck, but if your not get into the habit:
Pull out the engine stop and tap the starter. It should roll over if all the internals are still good, and the head gasket isn't leaking fast enough to have filled the cylinder back up while you were reinstalling the injectors. (I've had that happen) The reason you crank it over with the engine stop out is to check for hydrostatic lock. The military requires this step in their procedure for starting.
If it was coolent, you will be able to tell when it shoots out of the injector hole. If it was just water, check the intake for a leak that could have sucked water in other than at the mushroom. (unless you know that's where it came from)