How can I help my friend on her diet?

How can you help her? You can't. She doesn't want help. She's doing exactly what she wants to do.

If you don't want to eat at restaurants, just say, "No thanks, I'll eat at home. You're welcome to join me." Then she can decide whether she'll have that Big Mac or whether she'd rather have a healthy home-cooked meal.

You know how you can help her? By telling her that she has to be willing to make appropriate choices without anyone else supervising her or she will never succeed. True fact.

You can't get with her 24/7. Tell her you're proceeding without her and that you won't bother her again about losing weight barriers. Otherwise, this issue will affect your relationship. In other words, set her free from your mutual pledge, and take care of yourself. She's not ready now but might come aboard later.

All you can do really is talk to her, in the end its her choice. If does not want to make a lifestyle change its her issue not yours. You may need to just tell her nah I dont want to go out to eat or nah I do not want to drink tonight. You got to focus on your health that is what is important, if she wants to make changes with you then great. If not thats fine too.

She sounds kind of toxic to your personal goals. Maybe you should point that out to her and suggest that you don't spend much time together until she decides to commit to eating better. If that doesn't motivate her then you are simply wasting your time trying to help someone that doesn't want to be helped.