I think either with the empowered muscle you cannot play rugby without ACL, not even for the All Blacks. Imagine that professional rugby players get more shock on the tackles, and more stress due to their training and so on. Maybe they play with a partial tear of ACL, but definitely not without it. ACL it's very important in flexed knee position (it is designed to stop the joint to be dislocated in the anterior way when the knee its flexed, also contributes to stabilize the shear movement)meaning that, every jump or bend could result in disaster, or even at every tackle around knee area. As a ex-rugby player I know you much you love to play the game, but that's the reason why he should be careful with his health so that he could play longer. Regards.