At what stage is this appearing? Also, if it worked before & now it doesn't, what changed?
When he clicked either the Generate or Publish option.
And if you try it? What happens?
Maybe something has changed in the SSL settings - screen shots of what you're trying to do might help.
Nope, checked the SSL settings. They are identical to those of someone that can publish.
Doing a bit more research, I found this post from two years ago:
Apparently, somehow his file associations had gotten diverted. Looking into that now. When he reassociated .htm/.html with IE, he was able to publish again.