I'm sorry to hear about the issue you've been having. May I suggest the following steps: set the TV to the Standard or Movie picture mode; lower the backlight on their TV to 7. (Menu > Backlight > Adjust with Left and Right Arrows ) Also, since this problem can be caused by excessive heat, make sure their set is well ventilated.
Try this for a couple weeks. If the problem doesn't clear up, please let me know!
Surprises never stop with my new Samsung 60" 7100 series TV. Have "cloud" effect in corners, visible in dark scenes. Internet research shows this is a fairly common problem with LED TVs, and suggested solutions vary from having tech replace screen to crackpot ideas like loosening screws and rubbing affected areas with cloth. Set is otherwise perfect, with beautiful picture, so don't want to tinker with it dramatically. Would appreciate any suggestions to reduce cloud brightness. i.e, should I increase or decrease backlight setting from factory default of 12? Or anything else?