[jeroenhd]

They can definitely vote in most local Dutch elections, though that ability differs per EU member state. As long as you have a valid registration with the municipality, you're eligible.

The example given wasn't about casting their own vote, though, but voting for someone else by proxy (volmacht). For that, you need to take someone else's voting pass, a copy of their ID (may be expired up to a certain amount of years), and a form of your own, valid, Dutch identification.

That last part is where it went wrong: they didn't have valid Dutch ID so the vote by proxy was rejected.