5-pin cable to 3-pin cable

If you’re going to give advice to laypeople, you should at least mention that the other 2 “remaining” phases must be properly insulated. Simply cutting them off is not enough, nor is wrapping tape around them. Otherwise, someone might eventually lie next to the cable in the garage and not get up again... especially children love to play and grab everything.

Therefore, the layperson should not fiddle with it, but get someone who can do it... and above all, is allowed to do it.

I thought that was clear. He just wanted to use an already installed cable and not necessarily connect it all-pole in the distribution. I also advised him to consult an electrician.

Therefore, best to:
Advise the layperson exclusively to get a professional and not give any tips at all. This way, you are legally clean in the end.

A nice tip. But unfortunately far from any reality.

Then you also have to get an electrician to change the light bulb – who will come just as willingly as for connecting the cable from the OP. Which is usually not at all...

I am aware that according to NAV only a registered installation company is allowed to work on cables.

Nevertheless, the advice is far from real life. At least in this case.