My friend has had double vision for a while now what could be wrong?

Could be serious. Sometimes eye strain can seem like double vision, but if they are really having double vision, it should be checked by a doctor to make sure nothing is going on in the brain. It could be something like a tumor (not necessarily cancer) or even the onset of multiple sclerosis among many things.
Double vision. There are numerous reasons for double vision. I would recommend seeing an eye doctor for a complete evaluation.