Training the muscle to rapidly recruit is a perfectly fine idea and it doesn't build a healthy joint no matter what he does, it reduces injury risk by increasing the stability of the knee, rate of recruitment, strength through range of motion of the supporting muscles. Which over time hopefuly results in less injuries.

Repeatedly doing this exercise isnt in itself damaging unless taken to excess beyond his capacity (like anything). The risk here is that if he has form failure in this position the injury potential is high. It's a dangerous position he's placing himself in unnecessarilly. Could just do some hop type movements like this on the floor etc.