Well, I hope that she starts to miss her huge boobs when she is a B cup again. With a bit of luck, she gets herself a pair of tissue expanders (I assune they will take away a lot of excess skin so the skin that's left needs to be stretched again) and shows how they get inflated.
One problem with this: There's basically a finite # of skin cells to go around, so stretching, then cutting off, then stretching again, will very likely result in thinner tissue. This means an increased risk of necrosis. The flip side of this is the reduced elasticity of the skin.
It's this reason why when a woman goes for a tummy tuck, the surgeon basically checks to find out whether she's completely done with getting pregnant. It's not an impossible scenario, but doctors being what they are, they want the patient to have the best possible outcome. Getting a tummy tuck, then getting pregnant, then getting another tummy tuck... my guess is that there will be an increased risk of something unpleasant.