before giving any supplement it is important to get it checked clinically and assessed why is a person very thin. if there come no underlying disease then just a change in diet n routine can help her gain all the muscle otherwise if an underlying disease condition is making her lean and thin as said by you, then the correction of the disease is necessary. just as the words go, it is better to seal the leak than to keep filling the tank every time.
it would be advisable to visit a nearby physician and nutritionist.
till then, give ur daughter-in-law a proper nutritious diet rich in proteins
and carbohydrates. alternate days walk for 20 minutes.
Dr. Rooma Ramnani (email@example.com)