Did he buy or rent the apartment recently and from his own savings+income? Many such stories involve an old rental contract, a family-owned apartment or sizeable inheritance.
IIRC the housing market in the Netherlands 4 years ago was way more accessible than the madness of today so that's probably not a surprise that he could afford it. Ask him if he could have bought in the market of today please.
Smaller 2 bedrooms can be bought in the center of Amsterdam for €350,000 with a €35,000 deposit (10%).
How is that not viable for a tech worker?
To get one of those 600k apartments you'd have to flip your first apartment for the bigger downpayment, and that's gonna take another 2 years.
That 2 bedroom apartment is in one of the most desirable areas of the most desirable city in the country. Location matters.
That’s 2 years saving to then live where you want and then an extra 2 years to live where you want in the apartment that you want which is oversized for a single person.
Why do you need a 3 bed as a single person and what’s your ideal scenario? Do you want to be able to afford a house without a deposit, is that it?