I just learned that my cousin is taking a Mediterranean cruise on Cunard. My wife is telling me that they have an open bar & all drinks are included with the fare.
True or not?

Rich,

that is absolutely not true! The standard drinks that are included on virtually all cruise lines (water, tea, coffee etc.) are included on Cunard as well, but that's it.

Extremely unlikely. The only exception I've seen is a full ship charter, and I am not aware of these on any of the Cunard cruises this year. (Of course, I could be wrong.) Drinks were definitely extra on our Cunard crossing, and this is the norm, as Raoul states.

Which ship will they be on, and do you happen to know the date?

Some cruise lines do have where you can get open bar as an amenity if you have 100 people or more in your group. Of course, I'm sure that's built into the cost of the cruise.