I'm an old timer so have been working online since the late 90's - my opinion is to look at this all the same as you would any other business. Marketing and Sales really haven't changed much over the years, only the 'vehicle' in which we deliver the message has changed. So his 'e-commerce' business is really only 'a business' and you need to market that business to a specific target market of people who are looking to buy what he is selling. [moose hunting, if you want to bag a moose, you need to go where they are]
Using Social Media as one avenue to reach customers is currently a very popular one. Although right now, growing concerns over censorship by FB, twitter & google may result in Americans switching to other platforms or reforms by these companies so don't be surprised if just because these platforms are popular today, they remain popular - so you always need to be on top of WHERE TO FIND your customers. [constantly evolving]
FB Pages are currently free - FB will try to push you into paid advertising, but buying advertising is not the only way to get exposure. By creating, adding relevant and interesting posts on a regular basis and sharing these posts in other relevant groups [potential buyers] get you exposure. Now you can combine your posts so that FB & twitter can be done with one click so the real challenge is to provide GOOD content & get your message in front of the right eyes.
So first, narrow down who will buy what he is selling? keep narrowing down, then look for ways to get your website link in front of their eyes with enough *bang* they will be interested in visiting the website. - so your social media campaign will be narrowed down to forums, blogs & such that meet your target market profile.