Hard to kill ducks with a 20 with steel shot....but they work OK with that hevi-shot...so buy a modern gun, as no vintage gun will do well on hevi-shot.
Hardly anyone makes a modern 16 sxs. A vintage 16 would require bismuth for ducks. Spendy but it would work.
A 12 ga autoloader might not be a bad choice. Hard to load and unload though.
Have you thought about getting a 12 bore with a pair of 20ga gauge-mates? Her recoil tolerance will improve over time. And you can vary the loads to the game. Or you could get a set of 28 gauge mates for the 16.
Important to consider fit as well. Nothing worse than a sxs that does not fit.
So I agree with the poster above that recommended letting her shoot different guns before choosing.
