My truck did this, and it was the master cylinder that was out. it could be air in the system and needs to be vented, but unless you have done a brake job any time soon, its probably the master cylinder. wish the best!
98 TJ sport 4.0L I6 356,000 miles.
No Engine, axle, tranny, or t-case rebuilds.
Check out my 5.3 swap in progress here!
Pump the brakes with the engine off...if it doesn't firm up and hold position there's probably an internal leak in the master. Did he just buy the jeep? If he did there could've been some brake work recently and might just need a good bleeding.