Im sorta thinking... 80% chance he infects her by accident. Then he has to look after her eversince. Some quest to get her normal again or something. its just..... that was too easy. There is some catch coming. When things are too easy, you know something is going to happen badly.
Tho, under the hypothesis she gets infected. If he can make his arm grow big. Wonder if she could make her boobs grow big ( ͡° ͜ʖ ͡°)