I agree that is sounds like a meniscal injury, but I would have thought surgery is not necessary. Look very carefully at their cycling position (alter seat hight?) and change the loading to the medial joint especially in the symptomatic position. If they can run without pain the Wikipedia reference-linkmeniscus must fundamentally okay from my experience. I agree with Dace that fully SLR and slump testing is needed and even check the saphenous nerve over as it passes over the medial aspect of the knee. Look at the hip rotation/control especially at ninety degree flexion and change the loading pattern across the knee.

Steve
Hallamshire Physiotherapy