Depending on her age, her dosage of thyroid replacement medication, lifestyle, stress, weight she already is, time of year...there are many factors in the hypothyroid disease of losing weight.
I for one, weighed over 220 lbs and lost around 80 or so being hypo and undiagnosed. The body will naturally get to a plateau, then it is time to up the physical work, make it harder, and the weight should move again.
You say she is losing slowly. That is how it should be.