I am going to humiliate this overweight girl and tell her to loose weight.
Why do you want to humiliate her like that?
I want her to know how bad I feel about this issue.
It's good that you're concerned about her health and appearance, but that's a personal issue. It's not appropriate for you to comment on her weight unless you have a really close relationship, and even then, you would need to tread gingerly.
It's not your's or anyone else business.
Just like you don't want me to get involved in your own personal business, she most likely doesn't want you to get involved in hers. It's best to let people live their lives the way they want to when they aren't doing anything that is harming anyone.