Advise please

Hi, can anyone give me some advise from a data protection angle. My brother has recently split up with his partner, who is trying to force him to sell their house. He recently applied to the halifax for a mortgage in his own name, but they rejected him and sent a letter to his ex partner saying they rejected his application, who is now using this knowledge to force him to selling up.

does anyone know the rules over halifax giving details like that to her.

thanks