Ok if he flushed the cooling system, he has plenty of water already.
Block will contain water from the flush process. Heater core will also have some water in it. Plus the last bits here and there behind the thermostat, and bottom of radiator.
Its nearly impossible to completely empty the cooling system unless you turn the jeep up on its front end and let it drain.
Best bet is to put in about a 60/40 mix of antifreeze/water. When the radiator is full, then run it till the t-stat opens. Like said above, have the heater running.
If you have one of those cheapo coolant gauges from the auto parts store, cheak your mix to see if your good. Just suck up some of the mixed coolant from the radiator after you have ran the motor...This will tell you if your mix is within spec. If not, then you can play with it a bit by adding pure antifreeze to it to richen up the mix or more water to dilute it.
After you have the mix about right, the 60/40 bottle you made up is good enough for finishing up filling they system. Fill the overflow bottle also.
Usually you want about a 50/50 mix of coolant. But when flushing, you are running a garden hose with pure water through the system. End result if you don't check your mix could be a cracked block in the winter if your in a cold area like me.
Hope that helps...