DVDs don't actually last much longer than HDDs. If the guy is bound and determined to have his content backed up to something other than a HDD, tape is the only real solution.I don't think this is stupid at all as harddrives fail and it happened to me way back in the beginning. At the end of every job, I back it up to DVD and just consider it part of the job.. and I have duplicates.. one on DVD and one on a drive. I find that to be some level of comfort and come on people. It would take him maybe an hour or so to do this.. and charge a full hourly rate.
I typically don't try and nickle and dime for this stuff, but you should be making it worth your time. Either a set fee to archive and if it goes over that amount additional prices for each. If you also put it on the better DVD media it should last 50 years or more