It is rude to criticize people due to their weight
Sara has a friend who put on tons of extra weight which is clearly having a dangerous unhealthy effect.
Sara wants to help her friend lose weight.
Sara convinces her friend to go hiking with her.
Sara constantly comments on her friend's weight and nags her about it.
Sara's friend has a great time and loses weight as it becomes a regular activity.
Sara's friend becomes very upset by her treatment.