I'd say something like "I'm sorry, but I checked all DVD's before mailing them, and as it's been 6 months, I don't really feel there's anything for me to do about this. Have you tried getting the DVD polished?" Because honestly, it's not like you know what they've been doing to the disc, the DVD could have just gotten scratched or something. Stores won't take DVDs back after a few weeks, after all; why should you?