I'm pretty sure this is literal stalking. According to the law, AFAIK, the detective carries the risk of getting arrested for anything illegal they do, so they probably have very good lawyers standing by whenever.

There are exceptions to the law, though. In my country, you need a special permit to run a PI agency, which isn't just blindly given out to anyone. There's also not a lot of useful stuff a private detective can do. No installing of car trackers, no tracking of phone calls, no IMSI catchers, no recording without some specific exceptions (recording on the public road is acceptable, but recording things like a car's license plate isn't, etc.). There's also a limit on what can be shared.

If this came to an actual lawsuit, a lot of the evidence would probably be thrown out and a counter suit for infringing the victim's privacy would likely be successful, though the payout in a European court would not be huge. The goal here, though, is to intimidate and keep everything private.

With these leaks, the targeted hackers might be able to start a lawsuit against Nintendo, but only because they got caught.

Practices like these are why I refuse to give Nintendo any more of my money. I hope someone finds something violating the GDPR so Nintendo can be punished hard.