While I don't sell any of the brands you cite, I must say that your concern about aluminum is totally unfounded. A bicycle frame is made up of triangles, and there is no compressibility in the frame members whatever they're made of. You've fallen for folklore.

Inasmuch as there's any truth to the issue of some frame materials being "harsher" than others, this is definitely a non-issue for a rider as small and light as your wife. A small frame, with such a light rider, is essentially a completely rigid object.

Flex does occurr in the front fork, which is not triangulated, so fork choice can have an effect on ride comfort, though, again, for a lightweight this effect is likely to be negligible.

Ride comfort is much more affected by geometry, saddle type, and, even more important, tire choice/pressure.