There is a risk of server instability by not having free disk space on C drive. For example memory management and Windows being able to write to system files. You could set the NLB load to 100% for wfe2 while you fix the issue to prevent users experience any issues accessing wfe1. This will also allow you to reboot the server if necessary.
Confirm other files are not installed on your wfe server - e.g. if you have a session state SQL server on there, you may have SQL data files on your C drive.
Check your memory settings, for some reason your page file may be full and after a restart or for some reason it is not being cleaned up.
Check your SP logs and system log files have not got too large. Confirm the location of your SharePoint and IIS logs.
Obvious one, check your recycle bin etc. for any large files that may have been put on the C then deleted (and not purged from the recycle bin).