I've ridden to work quite a few times with a heavily loaded rear pannier on one side with nothing at all on the other side. It does not destabilize you. I never even noticed the difference except when leaning the bike against a wall or fence. A would be leery of carrying a large bag on the front because it will affect the steering. She could get used to the weight on the front but it will be more stable with the bag on the rear. There are baskets that will attach to the top of a rear rack. That paired with a bungee cord or netting would be a better choice.