The EXACT error message would be a big help. I’ve never seen or heard of an error message like this one.
You can do an “Archive and install” which will preserve her old system, and install a new one, keeping all her personal settings. You then must do all the system updates.
Do the kind of install known as an “archive-and-install”. All programs and user settings and preferences, bookmarks, passwords, emails, etc, will be preserved; but the core OS will be over-installed upon.
I wouldn’t do anything until you actually see the message.
If she is a novice user, she might be mis-interpreting some stupid web pop-up trying to sell her anti-virus software.
I second Beowulff. This does not sound like any error message I have ever seen on the mac. Tell your friend to take a screen shot of the message the next time it pops up (Pressing Command (the key with the apple symbol on it), the shift key and 3 simultaneously). This will place a file with an image of the screen on the desktop. If you post the text of the error message, we will be much better able to advise you.