Assuming a two umpire system (you did not specify) the tag up at 3B is indeed the PU's responsiblity. That being said, I would never 'point' at my partner. Who did the offence look to? Again I am assuming that they looked to the BU.
As the BU - knowing that it was not my call (as I was watching the catch/no catch by F8) at best, when my partner made no call, I would call time to consult with him. I would then remind him that it was his call and let him make it. Now if he did not see the runner leave early, then he has to call it Safe. He would also have to deal with the Manager (Rat) if it was an obviouis blown call. Of course, as any good partner would, I would be there to support him.
Bet he doesn't make that boo boo again!
