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.
Except... Khan hijacks Reliant and Genesis as normal. But not being able to lure Kirk in since he's behind a desk, he takes Reliant to Earth and uses the Genesis device just to kill Kirk. His reign on the New Terran Empire is glorious but alas short as the planet destabilizes and any escape is impossible due to the arrival of the whale probe. Once Praxis blows up, this leaves the Romulans as the dominant power in the region.