This really is the worst advice ever. It's a sure way to get you in Real Trouble when shit hits fans. This is not legal advice either, btw. Just someone who read the tiny letters in the mortgage contracts.
https://www.magnifymoney.com/blog/mortgage/lying-mortgage-ap...
Is the UK more tolerant of lying than the USA? It doesn’t seem so via a Google search:
https://www.onlinemortgageadvisor.co.uk/mortgage-application...
Now, of course, they might not find out, but that just means it is a perfect crime, it is still a crime.
How can one get a mortgage in Holland without being a resident/having income declared there? The bank will not loan a foreign resident in those circumstances. Did he pay taxes in all those countries?