Congratulations on getting that client! I'm not too sure about the competition with the fashion niche, but I would say that the best thing to do no matter what market or niche you are going into, especially for an ecommerce site, is to target as many long tail keywords as you can. Some of these keywords should be related to the different clothing brands as closely as possible, while you can suggest things to him or her like adding a blog to get exposure from the longer keywords related to fashion that aren't exactly 100% relevant to his or her site. Following best on-site SEO practices is a given, but you can try adding social media modules and try to get some likes or shares to increase their page rank that way. If there are other professional fashion ecommerce sites and such, you can request a link back or a partnership or anything to this effect if your client is okay with this. All in all, be creative about it.
The beauty behind long tail keywords is, while there may be tens of millions of people searching a very broad term like "fashion", the competition is hopelessly high and your client will never, ever see their site in the results unless they dig very deep. By then, most of the user base searching for that broad term has dissipated once the 10th or so page has been reached (It's very rare to even establish a site in the first ten pages when targeting a broad keyword). Many newbies make the mistake of targeting not only one keyword, but also a keyword that is very broad and competitive.
However, with long tail keywords, even though anywhere from 100 to 900,000 people may be searching the term, there will be very low competition for them, and because you are targeting multiple long tail keywords like this, that 100-900,000 people can turn into many thousands, to even millions very quickly if luck is on your side. If not, then they will start seeing traction in a few months.