I published my Nagios checks to monitor the open files counter for special users and for the whole system. You can chek it out on github and nagiosexchange.
check_open_files
https://github.com/wefixit-AT/nagios_check_all_open_files
https://exchange.nagios.org/directory/Plugins/Operating-Systems/check_open_files-2Esh/details
check_all_open_files
https://github.com/wefixit-AT/nagios_check_all_open_files
https://exchange.nagios.org/directory/Plugins/Operating-Systems/check_all_open_files-2Esh/details