Also remember the more grains/sweetfeeds you feed the horse the "hotter" the horse gets as well. Any horse in good physical condition and reasonable age should be able to maintain its body weight on ordinary hay assuming light riding. The more riding it gets the more energy it will need to consume. Some people feed horses grain etc to maintain a sleek, no "hay-belly" look but it is about appearance not health.
Personally I would wonder about a horse that needs grain to keep some weight on it. Could be teeth due to age or poor health or de-worming. I would suggest a vet inspection, horse passes you pay, horse fails, owner pays. That deal usually sorts the wheat from the chaff pretty quick.
I would also recommend you and your daughter do some serious reading about horses before you buy one.
Regards,
Dave.