There are no exercises that will repair an ACL injury. You can strengthen all the muscles surrounding the knee and the muscles stabilizing the hip and ankle. This will help support the knee and leg affected, but will not repair the ACL itself. In many instances the person can wear a brace as well, but there is a good chance that eventually some type of medical intervention will be necessary. Many people have ACL issues that they live with and learn to work around. Often these people will say that occasionally there knee goes out. This means that from time to time they put too much stress on the knee in a position that shifts the knee too far forward (usually forward) and the nervous system feedback reflex shuts down the muscles of the leg. This makes them stop using the leg long enough to move it from the position that is overloading the ACL.
If you or a client has an ACL injury, clear any potential exercise programming with their physician, surgeon, physical therapist, etc. If you do not feel up to working safely with such a client, refer them back to these people for treatment and exercise programming.