I'm going to tell my best friend she has to lose weight
Why would issue an ultimatum like that to your friend?
I am really concerned for her.
Commenting on the weight of others is very rude and inappropriate behavior. Only your friend can make the decision on what her weight should be since it is her body and therefore her choice. If you are concerned about your friend's weight and want her to be healthier, you should try doing more active hobbies with her like going to the gym.
Well I'm her friend not you. None of your business.
You brought me into this when you said something. She might have a negative reaction to what you plan on saying, I was trying to help you not make her upset.