"You cannot do an operation of this kind unless it's friendly," the source said, speaking on condition of anonymity.
Ferrero declined to comment.
Cadbury is currently facing a $16.8 billion hostile bid from Kraft Food Inc, and on Wednesday, U.S.-based Hershey Co and family-owned Ferrero said separately they were reviewing a possible rival bid.
They did not specify whether they would combine efforts or prepare two different offers.
A friendly offer in such circumstances usually includes a sweetener for shareholders and some continuity of management.
The source added that there is currently "a lot of uncertainty and any outcome is possible."