I'm with rowanwitch on this one. No one gets to be 28stone for no reason or just by eating the odd bar of chocolate.
Also, I doubt that she is unaware that she is seriously overweight, so 'telling her straight' that she has a problem is not likely to be particularly helpful.
The answer to her problem must ultimately come from within herself. She must decide to try to lose weight in whatever way she feels best for her...and/or ask for a referral to a psychologist/counsellor to work out her other issues. And she will have other issues.
Be her friend, be honest but tactfully so. Emphasise her good points and encourage her to make life changes in whatever way she wants to make them.
Tea and sympathy, but try to steer her towards change...after all, if she keeps on doing what she's doing, she's going to keep on getting what she's getting whether that is fatter and fatter or more and more miserable.
Good luck to you both, it's going to be a long road.