Post Nuke Cookies

A friend of mine administers a website with chat. Friend is having issues with someone changing verbiage on their website and even adding erroneous and/or nasty things in place of correct information. Now has found Post Nuke Cookie on PC. Understand Post Nuke is Open Source Software, and this cookie may be tracking admin login information. Is this correct information about Post Nuke Cookie? Is Post Nuke access for website hackers? What can be done to resolve?