A 16-inch torso is not that tiny (mine is only 15", and I'm 5'5"!)
Basically, any woman-specific pack (in a size small) will accommodate her; many "youth-sized" packs will also easily fit a 16" torso, and most of the cottage makers have a short-torso option which is 15" or 16" at the low end.
So, she can use just about any pack out there, as long as you buy the smallest size! Now it's just choosing what features/weight/capacity she needs, and making your final pack choice...
In this case, anyway -- size doesn't matter very much! ;^)