Golf Rules: Obligation to Mark an Opponent's Ball on the Green?

Question
Skip answered quite well that a player can ask you to mark your ball on the green at any time.  
My question is 揹o you have to comply if the ball is not close to his putting line??br>
Here is the situation: Player A has a 5 foot putt, straight up hill.
Player B抯 ball is 3 feet from the hole and on a line which is 90 degrees to the putt line.
There is no way that Player B抯 ball can be hit by the putt.

I am not asking whether Player B should be a nice guy, just whether he is required to mark.

Thanks  

Answer
Hi Frank,

Your situation is handled by Rule 22-2. See the rules online here:
http://www.usga.org/playing/rules/pdf/2008ROG.pdf

If a player feels a ball may interfere with his own play, he may have it lifted.

On the putting green, it is very standard in competitive play that a ball be marked if it is anywhere withing the vicinity of the line of the putt.

Hope this helps.

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Best regards,
Ken Tannar