Hey David, sorry for your troubles. We total know how customers can be when things do not go right even months later. While I do not have a solid answer on this, I would love to share some thoughts.
1. If you truly sold a perfectly good phone, there is no reason to take a return, none.
2. I can agree with Brian because normally when a phone has lines running through the LCD, this means a few things.
- The customer may have dropped it and the connector maybe loose or they damaged the LCD permanently.
-The phone is experiencing a software issues.
-Or the battery is swollen and my be placing pressure on the screen, backlight etc...
3. If the customer wants resolution on this she is going to have to factory restore the phone 1st to rule out any software, which at this point, with the Apps on the screen darken like that it really looks like it needs one.
Side Note: We have had many customers complaining about touch issues on 5 series and down since this update in store. Factory resets have helped, however one customer did recently get Error Code 56 by doing this and had to take there phone to apple. Which is definitely something this customer can do if the phone is all original.
Hope this helps!