I'm sorry I didn't see this earlier!

So basically your friend is dead-ass wrong, but you've fallen into error a bit as well.

Knights and men-at-arms were undoubtedly the most powerful troops on the field, whether mounted or afoot (which they did with some frequency!). They had the best equipment, the best training, and generally, the most willingness to kill. But they were not invincible and could be beaten if they behaved foolishly or were not supported by the other combat arms.

Medieval European armies were essentially composed of three distinct arms: infantry, archers, and cavalry. The best armies used all three of these well, and could overcome a weakness in one arm through skillful handling of the other two. For example, at Bourgtheroulde in 1124, a strong force of rebel cavalry attempted to charge a small Anglo-Norman royal army that had dismounted to block their path. The rebels were shot to pieces by the royal archers and never even made contact with the dismounted knights protecting them.

Spear-armed infantry could certainly be useful, though they were rarely decisive. If they kept their ranks and stood firm, they could resist even a heavy cavalry attack; if they broke ranks, they were shark food. Their lack of mobility and drill was a disadvantage on the attack, and they generally performed best when used as a blocking force. Both Byzantines and Crusaders countered Islamic horse archery by using the infantry as a portable wall, behind which archers and horsemen could shelter, waiting for the Muslim horse to tire, when they could charge out and rout them, as at Arsuf.