GDPR and the royal mail

[Hamdam]

I really couldn't find a more appropriate forum so forgive me if this is the wrong place ...my daughter moved home 3 years ago prior to which she had set up a regular payment to the courts for a motoring fine upon movie g she contacted everyone she could think of to give her forwarding address . Yesturday she received a message via social media from a stranger who now lives in her old home. upon open g the message she found a nasty message saying he was sick of getting her mail and a photograph of a court letter opened and detailing everything from her NI number DOB druving licence details etc asking if she was the right person ...he has presumably sent this same message and photograph of this very personal and data heavy court letter to every woman with the same name on social media he could find !! what can we do to stop this man opening any other mail that has slipped through and how do we limit any future reproduction and publication of her personal data
thanks for any advice