I don't believe Shimano makes different leavers for their shifters, even if they did, it would be nearly impossible for all but the most skilled bike mechanics to install as Shimano shifters are filled with springs, cogs, pins that would be a nightmare to reassemble.
Playing around with the angle of the shifters would be the easiest solution. Nothing says that what might be a weird angle for you might be comfortable for her.
Attaching some kind of a plastic extention? Kind of dicey. Definately avoid drilling any holes into the arm/paddle area of the shift leaver. If you do go this route, it might be a good idea to stop by a local company that works with plastics, they might be able to suggest a type of plastic and chemical compound that would literally weld the two pieces together.
Probably the best solution would be to upgrade her to a Grip Shift type of rotating shifter. Thumb length is not really a factor with a rotating shifter. Your local bike shop might be able to suggest a model of Grip Shift that she would be able to use without having to upgrade her rear derailleur. Not all models of Grip Shift are compatable with Shimano derailleurs.
Cheers,
Juba
