I am NOT a vet...but it sort of looks like seasonal flank alopecia. Not too much you can do about that. I would go ahead with the treatment the vet recommended though, just in case. Did he do a scrape for mites? That can present as just a balding spot that gets larger as well. It doesn't look like a food allergy to me (though I could be wrong). Most of the time when I have seen that, it presents as a sore spot or the skin or is itchy or inflamed in some way. That just looks like the hair is falling out. Especially if she isn't itching or anything.
I hope that made sense! lol