First question really is why does this happen? When he creates the link to the pdf files and you view the source (within the editor before saving the content) is 'resources' part of the path? If not then some kind of redirect or url rewrite is taking place and needs to be tracked down and removed. If 'resources' does form part of the path then you need to find out why. This is most definitely NOT default Joomla behaviour and is either being caused by an add-on, or perhaps by a setting. It might be an idea to check in global configuration to see if anything is visible that contains 'resources'.
If he is using a non-standard WYSIWYG editor it might be worth switching (temporarily) to the default editor to see if the behaviour changes. If he is using some sort of third party 'linker' plugin (usually a button beneath the WYSIWYG editor) try inserting a link manually, or using some other mechanism (specifics will depend on editor in use).
If all else fails - and ONLY if all else fails - the following line added to the .htaccess file should work. Add it after the RewriteBase line - but before the rest of Joomla's rewriterules:
RewriteRule resources/(.*)$ $1 [L]
I've noticed that some servers seem to require a slash before resources, and others not. If it doesn't work without, then add it.
Oh, and if you don't have a file named .htaccess in the root of the site you either need to tell your FTP client to show 'dot' files or 'hidden' files, or if no one has done so already rename the htaccess.txt file to .htaccess (starts with a dot and has no other file suffix).
