She should have no problem if she enters with a Spanish passport.
If it is shopping you are after, then certainly go to MeinZeil in the city.
The Kleinmarkthalle is good for some Mediterranean and German cuisine. You could always head to a butcher like Heininger, which has a great lunch menu.
Alternatively you could also visit Mainz which is not too far for a different, local vibe (certainly prettier than Frankfurt imho).