Pipe interrupter bathtub Grohe Rapido Smartbox

So strictly speaking, would I be liable if I advise him to install the pipe interrupter, and he doesn't do it?

You are liable for any damages caused by improper installation. Since you are a layperson, the entire installation is considered improper regardless of whether you have done everything technically correctly. The pipe interrupter is probably not even the biggest problem.
In addition, you are committing an offense (illegal work). I do not know what the violation of the master craftsman obligation and working on the drinking water network formally constitutes as an offense.
By the way, I am also legally a layperson, and my statements are theoretical in nature and do not constitute legal advice ... :p

Okay, got it.

Different case. This interests me now

If I now, for example, had a master craftsman business and acted as a professional. And advised the customer to install a pipe interrupter, but he doesn't want that (for aesthetic reasons), how would that play out?

As a clever sanitation master, you would of course allow a non-visible variant for a 30-50% surcharge. Or you would reject the improper execution and thus further cooperation, with a written notice that you have informed the customer about the pending but professionally necessary work. Ideally, they would then countersign it. Or you just do it that way and know that nothing will ever happen.

Bad.. well, I hope nothing ever happens, my conscience is just too strong.

What is supposed to happen there? The acquaintance shits in the tub, leaves the slurry water in it, and the showerhead hanging in the water, which sucks up the dirty water, and then he pours himself a glass in the kitchen for the dementia pill?