It is usually a flexibility or ROM (range of movement) problem at the hip - usually tightness in the quadriceps muscle.
But in the first instance get the guy tested by your physiotherapist or chirpractor. He may have a weakness there too, or he may have the top of the femur rotated, pelvis tilted badly.
Then you have some leeway to start a serious therapy: It could involve starting with some deep tissue massage which would loosen up (free up) the ITB (iliotibial band) and quad to restore length to the muscle.
Then when the muscle is liberated in the mechanical sense, you can also start some basic quad stetches - holding a lunge position gives you the basic idea …