How can I encourage my friend to lose weight?

I have a friend who's quite heavy. She complains about her weight all the time, and I think she has pretty low self esteem. The problem is she's never done anything about it or even tried to. The thing is she has a pretty face, and I know she'd look really good and be happier and more confident as well as feeling better if she lost weight, but I don't think I can say anything to her without offending her. I've tried telling her that I'm on a diet or working out or whatever in the hope that she'd join in, but she doesn't seem interested. Should I just mind my own business? I just hate to see her so unhappy with herself because of something that's fixable...
How can I encourage my friend to lose weight?
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