DOC, my major was in electrical/electronic, but as a controls engineer I had to interface with hydraulic systems, mainly opening and closing valves, turning on and off solenoids, and monitoring pressures that allowed the oil to flow and move cylinders on a piece of machinery.
Usually hydraulics are used to move heavy objects that require higher torques, using fluids like oil to do this, the problem is that the oil gets dirty and needs changing, just like in a car. Electronics are used to move equipment that require lower torques and more precise movements. There are pluses and minuses in using both hydraulics and electronics, and most manufacturers use a combo of both.
As far as the pontential job market for a hydaulic engineer, I think his school would have knowledge of this, or he could Google it.