Well, as I understand it, standard ROE allows for the mother ship to defend her crew - if aware there's even a problem, which is where the confusion is coming in...how far offshore was she standing? - up to the use of warning shots once permission is given, then one last warning, then, if authorized. Authorization can come almost instantaneously from CINCPAC, which the ship would be immediately in touch with. Which is why I said "or new orders".
But DID the Cornwall know? Were the sailors able to contact her? Again, how far away was she standing? If she were too far she might have tried to reach the location, but the incident was over by the time she arrived. This is pure speculation obviously. There's a LOT we don't know.