I think it is very unfair that a former coach can pass along information about a previous student. in business one has to sign a confidentially Contract and i think it should be done here also.
I agree confidential information should not be given by a coach who has actually coached the opponent such as tomorrow.
Seems there a bit of assumption going on here since I don't recall seeing anything about Gilbert passing "confidential information" about Andy to Nishikori. Maybe there's something about confidential information tucked away somewhere in the ITF's Rules and Regulations, but if there is then I would think it might be something along the lines of the National Coaching Foundation's Code of Ethics & Conduct for Sports Coaches, which states:
Sports coaches inevitably gather a great deal of personal information about performers in the course of a working relationship. Coach and performer must reach agreement about what is to be regarded as confidential information, i.e. not divulged to a third party without the express approval of the performer
However, there's a difference between personal information and tips about tactics. Andy isn't exactly an unknown quantity and I'm sure Nishikori will have watched videos of his and other opponents' matches just the same as other players do, so I really don't see what the problem is.
Also while Andy and Nishikori have never played each other in a singles match, he and Jamie did play Nishikori/Ito in Tokyo, beating them 6-3, 6-4.