Believe it or not, the self esteem can be improved whether the body is or not and that should be a primary part of trying to help her change. Encouraging her when you see things she does well, complimenting her on her hair, eyes, a particular shirt she's wearing (only if you are sincere!) and just being around for her can start the process of making her care about herself again.
The food issue is a tricky thing. You can't really bring it up without making her feel bad (and making her run to food more). You will have to wait for her to ask you how and show desire to get fit. Don't give her strict rules about what she can eat or she'll just hide it from you and that is a step towards a lifelong eating disorder. Here is what you can do:
Talk to your (soon to be) hubby about it. Make a plan together to eat healthy and exercise. Then when the types of food change in the house it can be because of YOU GUYS and your diet, not because of HER so the pressure is off her. It will take her a while to like the healthy foods so she'll complain, apologize, but say that you and your hubby are going to eat healthy and having junk food around is really hard on you guys.
Also, don't knock out all the junk at once. First start changing the types of junk to a bit healthier (whole milk to 2 % to 1 % to skim) Potato chips fried to baked and slowly until no potato chips are left. Full fat beef to lean, white pasta to wheat...white rice to brown ...you get the point. The slower you go the less resistance you will have. And never, never say you are changing things to help her lose weight. She will only lose when she wants to and that won't happen unless she feels accepted by you and you hubby as she is.
Hope that helps! My hubby was an overweight child and we have talked extensively about it.