The upgrade was automatic. It should just be in their library, as a separate entry, if they got it on Steam or GOG. Just before the release of the Enhanced Edition, GOG stopped the automatic updates (they were running a script to add the game in batches before release), so if they bought the original on the last day/hours it was available, the upgrade would need to be done manually. It doesn't sound like that was the case, though.
With the App Store version, Apple did not want to add the EE as a separate library entry, so it was included with the original release install, and a launcher program added to ask which version to play. In that case, they should just need to redownload and install the game.
