Many people do it but it's important to remember you're taking a risk and plan accordingly. As more people do it, authorities will prioritize trying to catch them and get better at detection and enforcement. Don't assume you won't eventually get caught, instead assume you will get caught and be resilient to it. Set things up so it won't be catastrophic or too costly if you have to suddenly relocate with no notice and also keep plenty of cash set aside to pay fines/back taxes. They key is avoiding doing things that make you easier for authorities to spot - usually financial transactions, living and travel arrangements. Also, you need to avoid doing stuff that brings any official attention your way. It's obviously best to keep relocating to another country every four to eight weeks with no more than 90 days in any country in a year but that gets tiring for many and that's when they get overconfident and slip-up.
To me, it sounds like your friend's approach wasn't very cautious. Keeping such a regular documented travel pattern and living in the same place long-term will look suspicious if your friend gets questioned by authorities, even for something completely unrelated like a loud party or traffic ticket. All it takes is one cop who's having a bad day making an extra phone call.
Obviously I assume he was following the advice of his financial professionals and reporting that income, but it could be the arrangement was hinky.
The outfit was one of the many Philips adjacent spinoff/startups that are so common in the Low Countries, so it’s possible it was either legal or they had enough political capital to not worry about enforcement on the Dutch side of things?
If you spend more than half of the year in a country, you'll usually need a very good reason if you want to avoid being fully taxable there (in some countries it's a hard rule, some look at the circumstances). Just taking a two-day trip every month looks a lot like deliberately trying to dodge taxes - such things can easily lead to criminal charges.
This does sound like short-term contract work where they didn't want to work through the channels (time constraints) so chose the most expensive way to do it.