Do the knots develop throughout the day, or after she sleeps?
I need a very hard bed(well, at least compared to most people) or my back hurts after sleeping. My bed consists of a futon mattress on the floor, lol. When we go to my girlfriends house, her soft normal mattress makes my back hurt.
If they happen through the day, it's obviously something that she does. Does she sit for long periods? If so, she needs to request a new chair, one that is fully customizable/adjustable, and play with it until it's better. If she stands, she should look into her shoes and posture.
I also recommend a chiropractor. They should be able to tell you whats wrong. I really have no idea why so many people here think they're quacks, because they really are not. There are people that are worse than others at any profession. If you don't like the first one you try, go to another one, and so on.