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Your client is probably thinking of some of that type of pages he's seen, but what's called a doorway page by some is not actually a "doorway" as such, but a normal content page that's part of a site. Those have the same look and feel, interface and graphics as the rest of the site, with the content, including text and possibly photos, targeted to describe and rank for a particular keyword phrase representing a service or product offered.
We don't do links to sites, and aside from some people not being willing to show their work, it could end up as a "talent showcase." ;)
Those pages can actually be an enhancement to a site, particularly those that have most of the site inside a shopping cart with dynamic pages and nothing more than product photos, a snippet of product text, a price and a buy button. They can be informational as much as promotional. Well done, they'll just look like any other normal page on the site.
There are also doorway pages that can be done externally a part of another site with a related theme, with a link to the main site, but again, those can be done to look "normal" with original text, graphics, photos, etc. and can often rank higher than the site they're done for, depending on the structure of the main site.