Despite all my qualifications, I have no explanation of how a magnet around a pipe can change the properties of dissolved and suspended ions and compounds.
I live in a seriously hard water area, you pour a glass from the tap and it says "who you looking at? you looking at my bird? outside!"

One of my friends also an engineer said this sounds mad, but those magnet things work.

IT ALL DEPENDS.

Some people bolt one on and nothing changes. Noone knows how this works so not a surprise.

It is not a placebo, kettle descale has gone from 2 weeks to 3 or 4 months, there is still stuff there, but it appears to self precipitate as tiny foggy granules in the water rather than scaling the heating element.

There are snake oil specials like one company that shows a coil around a copper pipe, hmmm that`s going to do a lot at AC as you claim. NOT! I ignore these as also pointed out by "which?"

Whatever the current orthodoxy,  I  have found that despite all my internal objections based on my education this does work, in some locations so It needs proper investigation. What is the difference in assay between where it works and where it doesn`t? no-one has done these tests.

I bought one of these because it was stupidly cheap and I was drunk.

Sober I have to come to terms with the results, and I cannot explain them.