Hi Michael,
You've gotten some excellent replies. I share Chris' sentiment about firing this client if they are telling you - their SEO - how to do SEO. As members have explained here, it has been many years since Google devalued the meta keywords tag, specifically because people were using it in the spammy way your client wants to. In fact, I would let your client know that not only will this tactic not help him, it could very likely hurt him.
Last year, Google updated their Webmaster Quality Guidlelines (share this link with your client: http://support.google.com/webmasters/bin/answer.py?hl=en&answer=66358) to speak against keyword stuffing of this kind anywhere on a page. The language reads:
"Keyword stuffing" refers to the practice of loading a webpage with keywords or numbers in an attempt to manipulate a site's ranking in Google search results. Often these keywords appear in a list or group, or out of context (not as natural prose). Filling pages with keywords or numbers results in a negative user experience, and can harm your site's ranking. Focus on creating useful, information-rich content that uses keywords appropriately and in context.
Examples of keyword stuffing include:
- Lists of phone numbers without substantial added value
- Blocks of text listing cities and states a webpage is trying to rank for
- Repeating the same words or phrases so often that it sounds unnatural, for example:
We sell custom cigar humidors. Our custom cigar humidors are handmade. If you’re thinking of buying a custom cigar humidor, please contact our custom cigar humidor specialists at custom.cigar.humidors@example.com.
*emphasis in bold mine
When Google makes a point of saying, "don't do this," and website owners ignore the warning, they are courting a penalty.
Share this information with the client and at the same time, lay out a proper Local SEO plan if they are, in fact, a Local business. Hopefully, this session of education will bring them up-to-date on best practices, but if they persist in insisting on spammy practices, tell them you can't serve them. You don't want spammy clients, believe me.
Hope this helps!