Pipe interrupter bathtub Grohe Rapido Smartbox

Hello dear home builders

I have a question.
I helped a friend install the bathtub. Now he said he doesn’t want to install a backflow preventer.
It would be too much effort for him.
The Grohe Rapido smartbox is about 20 cm above the bathtub, so according to the instructions, the backflow preventer must not be installed there.

This is too risky for me, do I have to point out to him that he absolutely must install one? And what happens if he doesn’t? Can I be held liable for that?

Regards

Can I be held liable for that?

Are you a professional craftsman? It's simply not a standard-compliant installation. But on the other hand, what could practically happen?

Are you a skilled tradesman? It's just not a standard-compliant installation. But on the other hand, what could practically happen?

No, I am not, I am a layman in that sense.

Then, in my opinion, nothing can happen to you, but if something does happen (completely independent of the [Rohrunterbrecher]) - e.g. a pipe damage - their insurance may possibly not pay.

And what if I received a small wage as gratitude for that?

No idea. Strictly speaking, you probably would be liable then. But if your acquaintance is so brazen as to report you to the guild or insurance, I wouldn’t have a problem if you say you didn’t carry out an installation, but merely cosmetic repairs. Then it’s one person’s word against the other’s.