It is a healthy climb of about a mile and a half and there are well over a hundred starirs to get to the top. However, there are benches along the way to rest and the incline is not so steep as to thwart someone not in shape. If you allow enough time, take water and a sun hat, she should make it to the top. The view from the top of Diamond Head is AWESOME and well worth the hike. Be aware that until you get to the top, the tradewinds are not cooling the temperature and it can be HOT!
Have a great trek on Diamond Head.