I, the owner of a private repository have a maintainer who forked the repository a while back. The repository was private when it was forked. I looked at their account today and it shows that the repository is public. Is it actually public or private?
If you can look at their account and see it, it's definitely public. Not only that, it's in their account, meaning they own the repo, not you, even if you legally own the code (see below and IANAL).
Once you allow someone to copy something (that is what a fork is: a copy), there is no way to undo that without their cooperation or by force of law. That's just the nature of giving anyone access. GitHub isn't going to give people the power to unilaterally delete repos in other people's accounts. Ownership is a complicated issue and they aren't going to want to get involved with that mess, or be in the role of cop. Even if they did, that doesn't solve the problem of local clones out of their reach, or the many other ways copies proliferate.
Your only recourse is a legal one. What were the license terms of the code? Who owns the copyright? If they were the maintainer, did they sign over all rights to you?
Actually you have another recourse: Talk to the owner of the other repo. Ask them to delete the repo. If they refuse, think about what you would have to do legally to compel them: prove copyright ownership in court. Now you see why GitHub wouldn't or can't get involved.