If she believes that you think the best thing for her to do is to hide under a tent, you'll never get her out of the house.
Do: Be considerate when snacking around her.
So, you're Mr. Metabolism. You can eat a large pizza and an entire bag of chips and then lay around doing nothing all weekend, and still not gain an ounce. That doesn't mean you should shovel in a bag of her favorite snack food while she's nibbling a carrot. If you're hanging out together and you get the munchies, go for the bag of Pork Rinds that always made her wrinkle her nose in disgust instead. She'll be glad she's watching what she eats.
Don't: Change your habits completely to suit her needs.
Not consuming mass quantities of her favorite food in front of her is considerate. But she shouldn't expect you to change all of your habits just because she's dieting. It isn't fair, and it doesn't do her any long-term good. Part of weight management is learning to make healthy choices regardless of what's going on around you. If she can only stay on track when you're dieting with her and caves the minute you break down and order a cheese steak, she's going to have a long road ahead of her.
Do: Suggest fun and active things to do on the weekends and evenings.
A big part of fighting weight gain is choosing to stay active. The cuddly couch potato routine is nice, but not great for the figure. Being active burns calories, tones your body and keeps you from thinking about food. Join a gym and work out together. Or just take long walks around your city, park, or hiking trail. Do yard work together. For evening outings, consider something like roller skating, ice skating or dancing instead of the more sedentary dinner and movie routine.
Don't: Encourage her to join a gym with you and then drool over all the hot babes in their workout gear while she's huffing and puffing on the treadmill.
I don't think this one needs any explanation.
Do: Express interest in her efforts from a health standpoint.
Let her know that you're glad she's trying to take better care of herself because you love her and want her around for a long time. Make sure she knows that you care just as much about what her new exercise routine is doing for her heart and mind as you do about how quickly she's going to be comfortable wearing a bikini again.