Welcome to the Pain Management Forum. Please forgive my tardy response. I am sorry to hear about your friend.
I am far from an expert but in my opinion, yes your friend's pain could be related to his recent knee. Our body is all one and connected. Often if we injured one part a corresponding part will also hurt. This can be due to a change of gait or the change in the way we use it caused by the initial injury.
However it could also be totally unrelated. If his pain originates in his big toe it may be that he is experiencing a painful gouty arthritic flare. There of course are other possibilities. It is important that your friend consult his PCP.
I hope your friend will soon be feeling better. He is fortunate to have you as a concerned friend. I hope you will let us know how he is doing.
Best of luck to you and your friend,