In order for your customer to get a low-e/argon storm door you would neeed to find a double pane door. To incorporate the argon gas you need a space between two glass panels. However, you can get a single pane storm door from Provia,
www.proviadoors.com, and they have low-e as an option on their glass panels. I used one on a Fairlington job this past January for the same reason your customer requested, to reduce the ulta-violet ray penetration causing fading. As a bonus, storm doors qualify for the stimulus tax credit.