Physiodude makes a point. In case you want to improve her exercise tolerance I would say though improve her core stability first. which in general, also in non 'handicapped' is not very good. By improving the core stability use of energy will be far better and therefor a possible improvement of her exercise tolerance. Try out yourself if you have a push bike. slump on your bike or tighten actively the muscles around the waist (e.g. abdominal) and see if it makes any difference to you.