Is she exercising or playing sports? Does she watch a lot of TV? Eat more than she was?
She may want to see a doctor to test for things like thyroid disorders which can cause rapid weight gain.
Yeah sounds like she could indeed have a thyroid disease. Graves disease in particular starts off overactive, and symptoms are usually ignored or not noticed...for me it was impossible to gain weight, I had way too much energy and difficulty sleeping. When the thyroid gets overworked though the opposite happens and you become underactive, very quick weight gain and extreme tiredness.