It's a tough one. A girl at my daughter's school is getting bigger and bigger and I know for a fact that she doesn't eat well. When the husband isn't home the mother doesn't even bother to cook a meal because she says her kids won't eat fruit or vegetables so there is no point. He goes away for work a month at a time and the kids live on take away and she has a twistie sandwich for lunch every single day + donuts, those chocolate dunkeroo things, chocolate muffins etc. I always have to bite my tongue when around them because all she ever talks about with her daughter is junk food (when mentioning food anyway...there's never any talk of anything healthy).
I just don't think there is a tactful way to say anything. I don't know this woman well enough that I could say something without her telling me where to go.