My friend has a pre finished gloss hardwood floor. The areas near the windows have changed color over time. They do not have a faded look. It apprears that the polyurethane has developed a cloudy/milky apprearance. Treating the surface has no effect. Removing the polyurethane with stripper revealed the original stained finish. A few of the boards in the effected areas are perfectly fine with no discoloration. Any ideas on what caused this? She would like to fix the problem with out sanding the floors. Any ideas on how to fix this with out sanding?

This is most likely cause by the ultra-violet light that comes through the window glass affecting the film. Some glass, special double paned, has UV protection that helps reduce the damage. Curtains are a less expensive option.
An area rug may cover the damaged area so it is not so noticeable.