There is no need of any case law Rahul ji, the law itself is very clear, a PP can be assisted by a private lawyer. You just go thru S. 301 CrPC.
301. Appearance by Public Prosecutors. (1) The Public Prosecutor
or Assistant Public Prosecutor in charge of a case may appear and
plead without any written authority before any Court in which that
case is under inquiry, trial or appeal.
(2) If in any such case, any private person instructs a pleader
to prosecute any person in any Court, the Public Prosecutor or
Assistant Public Prosecutor in charge of the case shall conduct the
prosecution, and the pleader so instructed shall act therein under the directions of the Public Prosecutor or Assistant Public Prosecutor, and may, with the permission of the Court, submit written arguments after the evidence is closed in the case.