Depending on how old the Subcompact is, it should have came with a short 10 round mag from the factory. The newer subcompacts have the short 13 round magazines. If he is looking for the full length 10 round mags that's probably going to be almost impossible to find in this area, I didn't know many people who owned these pistols before the assault weapons ban sunset so the low capacity mags probably aren't going to be plentiful in this area. His best buy may be to browse some of the pistol/gun forums.
Sorry to hear he's going to Commiefornia though, if it wasn't for their firearms laws and what not I probably wouldn't mind living there but I like my weapons too much.
Edit-if he wants to buy some if he can't find any tell him to check out http://www.pistolgear.com/proddetail.php?prod=XD0923
When I had my XDs I usually used them for new magazines and parts.