I'm going to make a girl feel fat and make her cry
Why would you want to make the girl feel fat?
She really wants to be fat and she's always eating a lot of junk food.
It's offensive to call someone fat and then make them cry. You're being rude to this person for no reason. Have you thought about being polite?
I want to make her feel like she should eat better, the only way she will get healthy again.
There are other ways, for one example you can give her some healthier foods. You can offer to exersize with her at the gym. Most of all you can offer some mindful concern without being so mean.