I am guessing there is 2 sitemaps because there is a plugin (Yoast's Wordpress SEO Plugin) installed which is creating one sitemap and then there is the default xml sitemap from wordpress. You do not need both of them, but until you verify there is no duplication between them you can just keep both of them. I would guess it is very likely that the URLs are the same between both of them so only 1 is probably needed. One nice thing about the plugin sitemap is that it creates multiple xml sitemaps broken up by type (post pages, category pages, etc.), which allows you to see the index ratio for each page type within Google Webmaster Central. So for instance if the category xml sitemap contains 20 URLs but Webmaster Central only lists 10 of them as being indexed, you have an index ration of 50% which is not very good. You could then look at the category pages to see if you can change them (add more unique content, etc.) so they are indexed and the index ratio increases. This can give you insight into what pages on your site Google sees as quality.
Yes he can definitely disable one of them, I would just advise him, before he disables one, to make sure that all of the URLs in the sitemap he is disabling are contained within the sitemap he is leaving.
Check out the screenshot below, it's of both my sitemaps on Goggle Webmaster, which should I keep? I submitted the second one only yesterday and today my traffic dropped a ton, could be coincidence but just in case, I've submitted both. Should I keep both or one or what?
Are all of the URLs in the sitemap.xml file also contained within the xml files listed in the sitemap_index.xml file?
If they both contain the same URLs then you can remove one of them. I would probably leave the sitemap.xml one since the index ratio (URLs in Web Index to URLs submitted) of it is much higher.