If the jetting is stock, the bike is very rich. When doing a lot of low speed technical riding, this can result to a fouled plug. Also, the stock plug is too cold for tight riding, IMO. Me and one of my buddies had '91 and '92 KDX250s, and with much leaner jetting and hotter plugs (we used BR7ES intsted of the stock BR8ES) the bikes no longer fouled plugs.
Underneath the bad jetting and stock fork settings, there is a decent bike hiding in there. With some attention to setup, the KDX250 can work pretty well and should be very reliable.
If the bike is flooded, he should twist the throttle all the way open and kick it through to try and clear it out (don't open and close the throttle - just keep it open).
Anyways, he should carry a new plug with him in case one fouls on the trail.