It lists the meta tags Google uses, and clearly states that all other meta tags are ignored. Of course the keywords tag is not listed, so you can categorically show your client that Google does not use the keywords tag.
"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.
A few bullet-points of examples follow, including one about lists of cities one is trying to rank for.
