Hi
From your question I couldn’t see what sort of reconstruction was patient undergo or what sort is his problem with a knee. Because there are numerous surgical techniques, using different grafts (from patellar tendon, iliotibial band, semitendinosus, gracilis tendon or even synthetics ligaments) and using different grafts is taking different postoperative problems. For example, patient who have hamstrings graft are prone to hamstring injury. Average time for rehabilitation after ACL reconstruction to return to sport was 12 months and now even shorter up to 6-9 months. So it is very important what kind of rehab program was he undergo until now? If you can ask it would be easier to track problem. There are few more problem who could be associated with ACL reconstruction like patelofemoral pain (on the injured or uninjured side), inferior displacement of the patella, patellar tendinopathy, soft tissue stiffness (arthrofibrosis) or soft tissue laxity.
I could give you full rehab program after ACL reconstruction but it wouldn’t help because that athlete should be back in sport by now. So try to find out what went wrong and treat that problem.
Best luck