If dolphins had a human-like facility for language, I imagine some captive dolphins would have figured out (probably through a discussion among themselves) that humans also have language, and that it would be greatly beneficial if they could communicate with humans. With both parties sharing this goal, I don't think it would take long.

On the other hand, I don't suppose human language emerged fully-formed (I don't think there is much evidence for Chomsky's views on this), and these pre- or proto-language skills may provide a lot of insight into its evolution.