Honestly, lots of us Dutchies leave the house from time to time without ID. I don’t even think to carry it when I go running, or to the gym. And most of the time if you interact with the police while out without your ID it isn’t really a problem.

But that is of course only reliably true if you’re Dutch. The right kind of Dutch, too. Autochtoon used to be considered the polite term. (I always found it rather menacing personally.)

If you have contact with a police agent and you’re speaking English or are brown, yeah, better have that ID.

Of course anyone can be in the wrong place at the wrong time - maybe the agents the OP’s friend approached were conducting surveillance and interpreted an approach by a stranger without ID as suspicious under the circumstances. But I have also simply seen lots of differences in how police decide to enforce the law based on whom they are dealing with.