Are you sure they are using Acrobat and not the free Reader, by any chance?
Yes, "Adobe Acrobat Pro DC 2015 Release (Classic)" is shown under "About". Thanks
In general, for licensing problems, you will have to contact Adobe's support: http://helpx.adobe.com/contact.html (don't use any of the "Forum" options, otherwise you will end up here again)
There are a few older questions about this problem. Take a look here:
(There are others, which you can find when you search for just the error message)
Thanks, I did look at similar posts, but something seems different about this problem. I persisted in logging out/in, & finally a notification that "my trial has expired" appeared. Next to the "Buy now" button was a button to license the software. Entering the existing license number fixed the problem & the user can edit again.