I wonder if you suggested to her to keep a food and exercise diary and helped her go through it weekly/fortnightly to suggest ways of improving things. Sometimes people work best when they have to own up to someone . . .
or you could suggest doing an exercise session or two together each week.
It's a hard one because until she decides not to give into temptation, or at least do it in moderation, there is not much you can do to help.
Definitely get her to go to the doctors to rule out any health reasons for not being able to lose weight.