Don't take this the wrong way, but your question is the equivalent of, "How do I change my Jet Ski to be a Snowmobile? What do I change on the throttle to make it run on snow?"
The line of code you want to change is a specific hook in WordPress. It passes the_content through the function wpmem_securify.
That function is designed specifically to filter the value of the_content and return results based on various criteria. You can't change what wpmem_securify is hooked to an expect a valid result.
If what you want to do is hide a link from non-logged in users, you can do that with the plugin's status shortcode (i.e. [wp-members status="in"]some text here[/wp-members] ). However, that does not prevent the link from being accessed if it is known. For that you need to prevent access to the file with something like .htaccess.