In your world that may be so. In my world we measure the voltage between L1-L2, L2-L3, L3-L1 and thus get 3x400VAC. It also says so in all the plans I have had in my hands in the last 15 years ;)
You may be more familiar with house installations than I am... but when it comes to industry, meaning "three-phase power" and when something needs to rotate, better let the professional handle it ;)
I really hope now that by "profi" you don't mean yourself. That would be almost a bit creepy.
According to Wikipedia, a professional is someone who does something to earn their own living. The way Mister posts here, it could quite well be his livelihood or he does nothing else all day.
Interesting news is coming from the financial world... some experts now no longer expect interest rate hikes, also due to the looming war in the Middle East, on the contrary, they even anticipate a reversal of interest rates by the ECB, meaning lower interest rates again.
So: Keep being nice and friendly. It's not difficult, he really is a good one. =)
Then I would probably just bring it up there quite neutrally. That you are worried about mold in the basement with the open window and the pipe and what might be done about it. From your story, I suspect he will find a solution. (Either he seals something himself, or the pipe is removed for now and only put back on days when it is needed)