Hi Jyoung,
I, as a manip physio, also experienced the same problem in my left knee. I found out not much in the literature but by personal clinical trial might be helpful. After I sprained my knee (meniscus injruy), I got posterior knee pain when I reture to play light intensity basketball. I found out that my quadriceps was weak and everytime after I play, pain experienced; and it was just come on in any occasions except resting. I therefore trained up my quad and after 2 months, I got no pain now. But if I really play very very hard game, the posterior knee "discomfort" will appear again. This might be because of my insufficient endurance of my quad. I, therefore, could only conclude weak quad could lead to posterior knee pain of muscular origin. Anyway, I hope this could help your client.
Cheers,
Kenneth