Based on the information provided, I'd say you can't. Let him waste his money and space.
My friend wants to buy a leg press machine, and asked me for recommendations. I said don't buy one, its a waste of money, and using one is a waste of time.
He insisted it's good for "training" because he "could put 4 plates on each side and do 3 sets of 12 reps, and afterwards, he felt really really sore."
How do I convince him leg press is a waste of time and money if he can already squat his bodyweight below parallel?
Based on the information provided, I'd say you can't. Let him waste his money and space.
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I answer all my emails: ALewis@StartingStrengthGyms.com
Tell him he will get stronger by just eating more meat. Then calculate how many steaks he can buy for the same amount of money as the stupid leg press machine.
Have you shown this person your results or other people's results doing starting strength?
Have you tried explaining the difference in force production or metabolism when doing low reps heavy weights and high reps low weights?
He should be able to understand that no matter what the number of plates is, if he can do 3x12, it's not heavy. And mabye explain that soreness is nothing but muscle damage and it has nothing to do with the actual adaptation of getting stronger, or bigger when it comes to medium to long term adaptations.
Walk him through the process. If he keeps leg pressing 4 plates 3x12 forever, what's gonna happen? Eventually he will have to either increase load or volume. How easily can he do that?
What about the rest of his body? How many times in his life did he find himself laying on his back and pushing something with his legs? Is he missing his arms?
Basically, as long as the guy has the patience to listen and you know what to say, he will understand, because this is not rocket science. If he doesn't want to listen, then no reason trying to explain.
In fact, when he buys the leg press machine, assuming you have a strong Squat, show him that you can do more weight or more volume than him without having done leg presses, that might spark something in his brain
Tell him 3x12 squats will make him more sore than 3x12 leg press.....sold!
Tell him that squats are more functional than leg presses.
Keep squatting and get real strong, wait till he's very impressed by what he can leg press, then walk in and do more then him easily. Then refer back to the previous conversation.
Just let him buy his shit. And let him learn wasting time, money and effort; as many of us here learned.
Sometimes, the red pill is taken in sips throughout life. And sometimes some people just never take it.
Update: I was able to convince him by showing him how much 45 lb plates cost, and since he doesn't need more than 1 pair of 45lb plates for any other lift yet, the leg press machine would essentially be a really expensive plate storage rack.