Nutrition feed?? Hmm.. If you can get the name of the food, we can probably help more. If it's a seed mix, rats can loose (or gain) weight, depending on what they're eating (and what they're not).
Is she a single rat? It's possible that she's depressed, as single rats get depressed and lonely without friends.
I'd like to know the food you feed specifically (that could be the problem right there), but aside from that, there's way to put some meat on a rattie's bones. Supplementing quality food with some baby food, baby cereal and/or Ensure may help. Also, giving Nutri-Cal/Ferretvite, a bit a day can help put pounds on pretty quickly.
I am a bit surprised that your mom knows she's thin but doesn't know how to "fatten" her up. Nothing meant by that, just musing! Do you know how much your rat weighs?