I completely understand both sides of the fence on this one- both your opinion and your husband's. It's hard to watch someone you love destroy themselves, and even harder to decide just what you can and can't do about it.

I will say that my opinion in this particular circumstance though, falls more to the opinion of your husband. I do hope that no one gets angry at me for saying this, since it appears that I am going against the majority here. But, if she were addicted to drugs or alcohol, would you still think that it would be best not to say anything to her? Would you still want to wait until she "wanted it herself"? I'm not bashing you, or anyone who disagrees with me, but I think that if you love someone, and you see them hurting themselves or others, you should speak up. Granted, there is a right and a wrong way to speak up, too. But, saying nothing to spare some possible hard feelings may not be in the best interests of your SD.
Since morbid obesity and early mortality runs in her family, I would think early intervention is key. If suggesting professional help is not an option likely to be taken seriously by her, maybe your DH could consult a professional on the best way to approach his daughter?
(((hugs)))