hospital advance payment for treatment

Hi,

My father suffered a stroke yesterday while working in Germany (he’s British but legally resident and insured in the Netherlands). He was transported to a specialist stroke unit and today was presented with a letter demanding a €5000 advance payment. Is this normal practice? I’m on hold to his health insurance company as I type, though he has basic coverage so not sure what will be covered. Never had to deal with this stuff but learning.

Here’s the letter: https://imgur.com/a/KrFWErO

Any help is greatly appreciated. Thanks for reading.