I coach boys HS volleyball and have advised some kids on lifts they should be doing off season to become more explosive, the big compounds are first and formost, with focus on powerlifting style. Heavy and explosive lifting. Plyometrics, jumping rope, wall jumps (she should know what these are).
Hitting power is mainly a function of proper form and armspeed, it really has very little to do with actual strength. Some of the skinniest kids on our team and other can crank the hell outta the ball. The process of hitting the ball is much more involved than a simple arm swing, with proper form the shoulder and torso will “open up” and then close creating a moving base on which the arm swing occurs. This is where you can add some real power, much the same way a javelin thrower employs rotational power into throwing in a linear path, or a golf swing transfers torsional power into sending the ball in a linear path.
The hitter should hop into a 2 footed launch platform, feet shoulder width apart, and use your body wheight to deepen the crouch thus causing more muscle tension and greater elastic response of the muscles. the arms should be slung backwards like a downhill shiers poles, straight and back, then upon accelerating up, the arms swing foward and up, aiding in the jump process and setting the arms in the correct hitting position. the hitting arm should be slung back into a cocked hitting position, again using the momentum of the upswing to drive the arm further back than it would normally go just raising it into that position. This causes more muscle tension again and allows for an elastic muscle response to accelerate the arm fatser. The non hitting arm is held out in front sort of as an aiming device. It really improves contact with the ball to “size it up” with the non hitting arm good ball contact also affects how hard the ball will hit the floor or the defender. ( I like to visualize it like my body is a gun, with my left arm being the scope, and my right arm cocked back in the hammer position like a real gun would have. then when I feel the target is in my crosshairs, I fire the gun and this also helps to create the feeling of explosion. So when you pull the trigger, (your shoulders should have naturally opened by having one arm in front and one behind you), you drive the non hitting arm backwards, and the hitting one should unleash like a whip (Think of a pitchers arm mechanics), with your hand being at the end of the whip and generating tremendous speed right about the time it is approaching the ball. the rest is history, your eyes are locked on the ball, your hand is on a terrorizing path to collide directly with the center of it, driving it down in a fury of white leathery hell.
Ok so maybe I got a little carried away, but she should work on her hitting technique as well as getting stronger overall to hit more powerfully next year.