My point is that the word "bad" is very subjective. What does the picture look like? Like I said, my experience with HDMI is either there is a picture or there isn't. I've even had it flash on and off constantly every few seconds. No such thing as bad picture. Maybe there is something wrong with their TV which makes me wonder why are you blaming the HDMI cable or even you? Has this same setup ever worked before you touched it?
I understand that to go out and buy a new HDMI cable even to test is going to cost you money. If it were me I would want something more to go on than the customer saying that sometimes there is a bad picture. I would have to see it for myself at least, otherwise I couldn't do anything. There are so many places this could go wrong I would just be spinning my wheels otherwise. Does he have a smart phone and could he do a video of the TV screen at least?
-Hal