And show them this page, also from Google:
https://support.google.com/webmasters/answer/66358
From the page:
Irrelevant keywords
"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.