As far as I know only upper level Shimano (Ultegra and Dura-Ace) are double or triple specific. Up through Shimano 105, the shifters are compatible with either. All Campy shifters are compatible with either. It depends on what Amatron has on her bike already and the age of the components. Even BB and front ders can need to be changed if she just changes her chain rings. The curves need to match.

Yes, freqently single shifters can be purchased.

Of course, there is always the possibility that she does have a triple crank without the inner ring mounted. You do see that. You also see doubles run with only one chain ring (I've done that). A mfg could have left off a chain ring and sold it as a double, or a previous owner could have removed it. A few cranks just happen to have bolt holes in the right spots; those are uncommon but not unknown.