Best thing would be gas engines as they're more broad range unless she wanted to work for a dealership that works on semi's. Pay range is good for experienced mechanics but hard to get your foot in the door with no major professional experience. To be honest she might not even be able to do the job of a mechanic whether it's gas or diesel simply due to the weight of the stuff that she would have to be lifting and moving. It's like women in combat there's no doubt that they are as smart and agile as the men and can shoot as good if not better but when it comes to having to lift something very heavy they don't possess the physical strength to do it. If she wants to become a mechanic get some sand bags and stack them up to weigh about 150lbs then ask her to pick them up and move them 10 feet all at once. If she can do it then she wouldn't have a problem as a mechanic if not then best to stay where she's at and become a shadetree mechanic.