[Plugin: WP User Frontend] wpuf_list_users causing memory issues

I’m having trouble with this plugin running on a site with a LOT of users (40410 users to be exact).
WPUF is causing all ajax requests via admin-ajax.php to fail due to the following error:

[08-Jun-2014 21:29:19 UTC] PHP Fatal error: Allowed memory size of 67108864 bytes exhausted (tried to allocate 524288 bytes) in /Users/tom/Sites/frenchentree/htdocs/wp-content/plugins/wp-user-frontend/wpuf-functions.php on line 392

The error is related to a function named wpuf_list_users() which as far as I can tell, is only ever used to populate the dropdown in the wp-admin area when selecting an override for the post author. Is this function used anywhere else in the plugin and if not, can it be changed so that these types of requests check the following conditions before running?
if( !defined('DOING_AJAX') || DOING_AJAX == false) { ... }

I could be mistaken, but from what I’ve found this would improve the code efficiency and solve the issue I’m having. I’d be happy to go through the