If your friend is a genuine anorak and interested in digging into data sets he will surely have the data showing a minimum sample size to draw any conclusions from. A sample size of anything less than 6 figures when the rtp is based over 8 or 9 digits isn’t even worth looking at as the SD or VD will just be too high.
nb I wasn’t a gto player or even very good I just spotted patterns easily and worked hard. I haven’t played seriously for over 15 years but when I did play I played to win and stuck to rules I made myself and later had others playing to my rules. I found it quite easy to win because back then players just enjoyed playing and didn’t seem to be too interested in winning – when players got better and it stopped being easy to win I didn’t play anymore.
I did work with hand histories from 100 plus players to decide how I should play certain spots and learn patterns- unless a pattern showed a profit over xmillion hands it would not be included in any system we played by… this is why I keep saying sample size is everything