I work in sports medicine for a Division I football program. I work with several ACL rehabilitations a year. The pre-habilitation you’re helping your friend with is crucial to his post-op recovery. I instruct my athletes pre and post surgery on Kelly”s cues bracing sequence (pg 29 Supple Leopard) and chapter 4 regarding laws of torque. Even without an ACL the gluteus and external rotation torque stabilizes the knee and helps stabilize the foot/arch. We then pattern the air squat mad deadlift patterns or category 1 movements.

As the patients ROM improves and strength increases we can increase load.

In short the lack of the ACL shouldn’t affect the stability of the knee in the squat if the person stabilizes from the hip.

From my experience and thanks to KStarr’s concepts this is what we’ve found in our ACL rehabs