Check riding position. numb hands is a sign of a nerve pinch somewhere.
My guess is either the handlebars need to go up, the bend of the bars may be wrong, or she's got the "newbie death grip" going on.
Newbie death grip: every rider has this...it's when they first start riding, and grip the bars way more firmly than needed...this compresses the tissue in the palm of the hands, and makes it far more likely for numbeness, etc. Grip should be enough to keep good control, but not tight.