If you want her to gain weight try the high calorie meals. My sim ate 5 in a row for 3 days and was near peak "fatness" of the fitness scale.
(Some calorie counted recipes listed here) https://forums.thesims.com/en_us/discussion/876976/calories-in-foods
Yoga is also an option I believe they patched so you didn't become a bodybuilder from stretching
And if you're okay with mods, mcc let's you "freeze physique" so they can work out without gaining muscle or losing weight.