Cheap and reliable don't necessarily go hand in hand. If your sister is looking for a 500 quid car that might run for a couple of years, best to get one with full mot for starters. I'd check ebay and Facebook marketplace. Check mot history on the .Gov website too.
I think the days of 500 quid cars are over aren't they? We drove past a garage the other day and I don't think there was a car cheaper than 2,500k. Used car prices have gone mad.