Can he walk? There are plenty of single-leg exercises that require very little (if any) weight if done correctly. There are also machines. Leg extension, leg curl, hip abduction/adduction/flexion/extension, cables attached to the ankle… Back extension equipment can be used to target the glutes and hams. Nearly every commercial gym has that hip extension/glute machine.
He can pretty much do anything besides make excuses if he wants.
Has your friend figured out drop sets, partial reps, rest/pause, or any other intensification methods for his arms?
I’m sure he has which means he’s shown that he can learn new things about lifting either by reading or watching other people at the gym. The fact that he hasn’t applied this to leg training shows that he doesn’t want to train legs and his foot is his scapegoat.