But I don't think fucking around with the rep scheme is going to do you much good if you want maximal strength.
For large compound exercises (exercises that use more than one muscle or joint) I think sets lower reps (1-5) are the best in terms of maximal strength development. For every muscle group it doesn't matter.
But...For stabilizing muscles like abs, obliques and back it's not clear whether you want lots of reps or fewer, or even train them at all if you're lifting heavy they're getting quite beat up.
Abs and back function differently in everyday life and sport than other muscles they mainly stabilize your trunk, while your legs and torso do the pulling and pushing.
It's unclear if you want them to be strong or to endure as much as possible.
But I'm no pro man. What I know is the result of some research that's all.
Maybe someone who knows more can teach us a few things.
Now this is one strong ************.
(That's not to say your friend doesn't know his ****, "fitness" is a blanket term, yes you will be more fit if you do 20 rep squats, but more fit for what? Maybe not stronger but you will have better endurance, it all depends on an athlete's needs in my opinion, I was talking about maximal strength, but there are other types of strength to consider, explosive strength and strength endurance for example)