If they are interested in working on a treadmill, that works very well also. Any kind of exercycle. I favor a lap pool and I have one in my house. I think that swimming is one of the best exercises for maintenance of cardiovascular as well as musculoskeletal health. Motor control is paramount.
Weakness is due to inhibition.
Your chore is to find out where is the inhibitor. You cannot strengthen a muscle that you cannot find and control. Each exercise program must be individualized for that athlete or that training for that sport.