If your goal is to keep her dry, would a pair of high wellies and a long coat suit her better?
I doubt if the snow will come up to her thighs, and from your description of her, it doesn't seem like she will be rolling around playing in the snow.
She's probably too afraid of getting videod rolling around in the snow and having it posted on youtube. Not that I would do that sort of thing.
The overtrousers are going to be better, especially since most act as wind breakers. She already has a heavy/waterproof coat.
She really needs the ability to try the clothes on beforehand, but given she lives in Mobile, AL, and it's about 60F this time of year and not the rainy season, the local shops don't have anything she could use.
If she was a more conventional size I'd just take her to the local places over here. But I'm an 18 with a size 8 shoe, and I have enough problems finding adequate clothing.
(Which aside from a health standpoint, I'd love to lose weight to have a greater selection of clothes.)
Oh well.