My wife finally asked me to do it for her, too.
I forgot how I did, but turns out it is very easy.
- Visit myactivity.google.com
- Search for youtube history section
- turn off or pause it
- I then also had to click the history tab in her youtube to delete past history.
And voila, no recommendations anymore, no Youtube rabbit hole of crap.
This is what she sees now when visiting youtube: