I would warn them, though would probably go to Spock and explain the situation to him in a calm and thought out manner, so that he could then deal with Kirk.
Ilia would be saved, Decker wouldn't need to sacrifice himself, V'ger could be stopped before it got into Earth orbit (thanks to the sacrifice of Ensign Ricky). Afterwards, Kirk would go back to HQ where he could get doughy behind a desk, Decker would resume his rightful command, Spock would return to Vulcan, McCoy would return to his civilian life and the new Enterprise
crew could head out on many wonderful adventures.
Commander Austin Harris, First Officer, Deep Space Nine (by FltCpt. Bossco)
8.01 - Darkest Before Dawn
(Chapter 8 added, 12/09/2015)