I'm going to make a fat joke about a friend who frequently laughs at such jokes. Now, I've this joke directed towards him.
Why do you want to make jokes about his body?
I really dislike his body, he should put efforts to lose weight.
Just because someone laughs at a joke, doesn't mean they enjoy being called fat. You could be hurting your friend on a deeper level. Losing weight isn't easy for many people, be considerate about your friends.
I'm not making fun of his body! I'm making jokes about him.
How would you feel if someone makes joke about your body? I guess you won't be ok with that. It's sensitive to make fun of someone so you should avoid it.