Inside or outside of the knee? Might just be a small bursitis type issue as the hamstrings rub on the bone at the insertion points. If so it is a mechanical issue and the bike setup needs to be checked. Tight ITB (therefore gluteals) need releasing and strength to balance out any over dominance that they are playing. No to surgery. No to other invasive techniques. Rest, frictions, biomechanical correction and time. Lots to consider on the biomech front.