Because generally when people talk about getting strong they are talking about max weight, low reps. And training to maximize low rep work is different than training for max muscle.
Adding muscle is going to make you stronger, but not always in the way people mean when they talk about training for strength. Which in large part includes nervous system training and movement training as much as building muscle.
For example, a big difference between something like bodybuilding and powerlifting is that A bodybuilder may use bench to train his chest. If it isn't hitting his chest the way he wants, he may change setup, switch to a variation, change to a different rep range, pre fatigue something, or drop bench all together in favor of a better chest targeting exercise, REGARDLESS of weight involved.
That isn't at all why a powerlifter benches. He benches to get a bigger bench. He will choose reps and variations and setups to address weak points in the movement, NOT his build. If he can make a tweak to add 10 pounds to the bar, he makes it, regardless of how it affects his chest activation.
This difference in basic mindset ultimately can change pretty much everything in a program.
Now, that said, neither the OP or his friend sound anywhere near advanced enough to start considering most of that. Other than, you need to train what you want to be strong/developed. If you want to be good at low weight compound movements, then make sure you do those and put your effort there. If your goal is a big chest then make sure what you are doing hits your chest hard.
So really, pick a tried and true program and make sure you train what it is you want to be trained. Oh, and eat a bunch.