What kind of fiber it is?
Acrylic or mostly acrylic fiber might tighten a little, but it won't shrink permanently. Some wools felt so much she might end up with toddler size mitts. If she chooses this with wool she must check them often... like every 10 min or so. If they are washable wool they shouldn't shrink much. Some yarns probably shouldn't be washed at all...say silk.