Hey, one of my players is playing the UA Artificer, and the most recent revision posted by Wizards includes spells from Xanathar's, however the DnD beyond version of Artificer does not include spells outside of the PHB. Is there a way that he can add the spells, even if they're technically from a different class's list?

The new artificer updates are slowly being added (I think spells were added today).

Normally, you could use homebrew subclasses to add spells, but you can't make a homebrew subclass for artificer as it is subject to change.