First of all, impress upon her how important it is to remember that 4WD is *ONLY* for low-traction situations: Snow, ice, gravel, dirt, grass..... The tires need to have an opportunity to slip, as the front and rear axles are completely locked together when that lever's thrown to 4WD (hi or low). When she turns, the front wheels will travel different arcs than the back, which means they'll make a different number of rotations than the rear, as they'll be travelling a different distance. If neither the front nor rear wheels can slip a little to compensate, the whole drivetrain will bind.
A slight bind will make it difficult to disengage the 4WD. A major bind might start snapping axles or ring gears or whatnot.
BAD GIRL! BAD, BAD GIRL!
Jacking the truck up until 1 wheel is completely off the ground is the best solution I'm aware of for relieving the bind. I've also seen someone dump sand in front of a tire and drive over it. Short of that, she may get lucky by straightening the wheel and driving forward and backward several times.