I have. Better than FaceID and under screen but I still prefer on the back. It had other benefits and just felt nicer than in the lock button

If it doesn't have to be on the back, he can just buy a phone with a fingerprint reader on the side today.

I see what you mean now. Having owned both kinds (and under-screen), I think they still have a point though - on the back was (slightly) better and I wish they'd come back.

I would personally rank a traditional iPhone home button sensor and the backside sensors on certain Android devices as equally great for unlocking while sliding the phone out of a pocket and general convenience.

Side button sensors work OK, too but I have much more misses on my supposedly more "modern" side button sensor phone than I ever had on old Pixels or any old iPhone with a home button sensor. I assume this is due to the size and general shape of a side button in comparison to an iPhone-style home button or old-Pixel-style back sensor which are bigger, indented and finger-guiding.