Even better...
Go to the local Police Station, with all the facts and the Denuncia in hand, and then serve a Denuncia on the Family and the Police Office who served the Denuncia on the Refuge.
At the end of the day, the Refuge is not braking the Royal Decree on Dangerous Dog's, but the owners are and I bet they didn't inform the Police that they didn't have the required paperwork.
We had to get all the paperwork for our Tyler, who even though he was a Rottweiler, was the softest big dog you ever did see... and we moved from the UK to Spain, and still got all the legal paperwork sorted.
So this, I assume Spanish Family, will know all about what they should have and will also try and get away with it.