if she really wants to lose weight, help her anyway you can.
personally, i'd suggest going to the gym yourself (excercise is never bad regardless of what shape your in) ask her if she wants to get a membership and go with you. if your doing it for you, she'll most likely be more motivated as it would be supportive to you. also, cook for her
not only does this earn you points, but it makes it a lot easier to monitor what exactly your putting in your body.
exercise is good for the mind as well as the body. it can keep a person from becoming depressed, and will help ensure self confidence. its one of those things that once you've started, its easy to keep it up.