If you have the time and are a decent writer with some basic research skills, then writing content, TITLE tags and META description tags for product and section pages and such can be a great help. Often the store owner is just far too busy to take the time to do these things. Stores end up using boiler-plate content and head tags borrowed from the manufacturers of the products they offer.
The problem with this is that the major search engines recognize duplicate content. Using the cut-and-paste approach ensures that whoever used that particular content first (in the search spider’s eyes) gets the credit for it and all the :me too” sites get none.
Past that, you can blog about what his store is up to, or participate in discussions about it on social networking sites. Just review the site’s spam policies first and be sure to always err on the side of caution, as people will often yell “spam” even when you are well within the site’s terms of service.