My experience with swimming is that simply being in water strains your lungs more (as you have to displace water to inhale air).

That's probably where the long-term benefits of lung capacity come from. Aquafit (where your head doesn't go underwater) is pretty low-intensity compared to on-land aerobics, and you don't overheat so much so it might be preferable for her. On the other hand, something like breaststroke is a good way of training yourself to breathe rhythmically while exercising.