According to Canon Law a baptismal sponsor has to be over 16 (though this can be varied) and a catholic "who has been confirmed and has received the blessed Eucharist". (canon 874 §1, 2)

The letter of the law which the Spanish Bishop is obviously sticking to is that "A baptised person who belongs to a non-catholic ecclesial community may be admitted only in company with a catholic sponsor, and then simply as a witness to the baptism (Canon 874 §2). So yes technically the man in question can only be a witness not a godparent. In this country I think many
clergy take a more liberal line in the interpretation of this canon or don't even know it exists!

Rev Robert Corrigan
Bishop's Private Secretary