Unbelievable.
Perhaps your union should point your management towards the UK legal requirements which are placed on both employers and ATC Licence holders (known as SRATCOH).
It can be found in Appendix D
here.
I believe the case you mention would be illegal under UK law.
It was brought in to prevent fatigue and human factors problems associated with ATC and shift working. As we are all supposed to be aligning in Europe, maybe it's time the Spanish government got on board with its safety obligations ?
If I was the lady in question, I think I may have developed a stomach bug an hour before my night duty and let the management pick up the pieces.