In the summer, I close some of my downstairs vents and make sure upstairs is open to get more cold air upstairs. It sounds like that may have been done there as well. In the winter, you need to reverse that. It's also possible they did this with dampers in the ducts rather than at the registers. Tell her to look for levers maybe down near the furnace. They're open if the handle is along the length of the duct, and closed if they are across it.