Originally Posted by FlyingCow

Back in the day, it used to be possible for people waiting on the resolution of their NIE extension to request a ‘Permiso de Regreso’ which allowed one to travel outside of Spain and be able to return with no issue. I remember using it, though for travel outside the EU, and with a return via AMS.

If this type of paperwork is still available it could be a good idea for your +1 to obtain it (it was quite straightforward as I recall).