Having “pain” while walking downstairs is very common. Running and walking downstairs are two completely different actions, does his knee hurt when he runs?
Possibly suggest he turns sideways when going downstairs.
Foam rolling and strengthening his quads/hamstrings would be great.
He should get his knee checked out by a doc also.
I assume the pain is in his knee(s) only. Does he have any pain during or after his runs? If he does, he needs to find what is the underlying problem and is causing the pain. Laying off may help him get over it – until he starts running again and it comes back. Things for him to consider – shoes, tight IT band, maybe have to look into orthotics, better stretching, etc. If he is not getting pain during or immediately after running, the pain may not be from his running and it could be something from else. If he is mainly just a runner, I would encourage him to cross train more – other cardio (swimming if it’s an option), strength training, core work, balance, etc. This will help and should improve his or anyone’s running.