You can lead a horse to water. . . errr. . . . a cow . . . . to skim milk. . . . wait, that didn't work. . . uhm. . . OK, that's so not going to work I can't even come up with an analogy. I mean as soon as you turn around she'll be hip deep in bon bons. Now if you want to help your thin GF/Wife/SO stay thin, keep her TFA from her fat friends: http://heartiste.wordpress.com/2013/07/19/the-greatest-gift-you-can-give-your-girlfriend/
Keeping her away from the groceries can be difficult. You might try hiring a rodeo clown to distract her while you bring the groceries in from the car.