737Jock:
Better read the NL-CH dta again then faust. Bet you don't have a single tax declaration definitive.
I think his residence is Poland, which, like Hungary, exempts the income taxed in Switzerland from further taxation.
Netherlands / Switzerland different story = tax at source is 0% and all must be paid in Holland.
Local contract or Confair is not up to the employee - it is decided by the company in most cases depending on your base. In rare cases you have a choice.