€120.000 a year puts him comfortably in the top 1% earners: https://www.cbs.nl/nl-nl/visualisaties/inkomensverdeling, but he complains he'd earn twice as much if he lived in an area where the cost of living is much higher?

I mean he lives in Amsterdam, which is more expensive than the rest of the country, sure, but nowhere near what it's like in SF according to https://www.numbeo.com/cost-of-living/rankings.jsp (Amsterdam is ranked 75, vs SF at 8), and I'm not sure if that index includes health care and social benefits like state pension, etc. Speaking of, that's only salary, ìt excludes stocks, bonuses, pension contributions, and other benefits.

Then there's accessibility; living in central Amsterdam means he can fulfill most of his daily needs and transport by bike. Can't get much cheaper than that.

TL;DR, he can move to SF if he wants to earn SF rates.