I have seen plenty of hacks on bikes that are meant to transform a one person whip into a ride capable of transporting two people. This was a new one for me.
The hacker took an old seat and some plumbing clamps and voilà.
With the footpegs less than a foot below the seat, this is a solution that the passenger would most likely outgrow.
Here in the UK these were commercially available, my parents used to have one. You would get the small saddle plus ditting clamps, and also the footrest.
I had one of these on my bike as my kids were growing up. I had the footrests on the fork though.
there is a risk that the passenger swings their feet into the front wheel – maybe having straps might prevent this. It was nasty when one of my sons did it.