If the person is fat, then they are factually correct. There is always a 'right' to say something, if it is factually correct. However if it is a moral action, it depends whether they are saying it to be gratuitously rude, and to bully or humiliate that person; or whether they're saying it because they want to help.
Most people who call someone fat don't say it do their face, they talk about them behind their back. This is simply engaging in scurrilous gossip, and while they have a 'right' to do it, it serves no purpose other than their own entertainment. They are aware it will upset said person, even if it is factually correct, and therefore it's a mean think to do.
Those who do call someone fat to their face, in an attempt to bully them, have no interest in them leading a healthier life. In fact they are likely aware that if they incessantly bully said person, they are more likely to eat more, in an attempt to feel better. This is certainly a nasty thing to do.
People who are trying to help, like a well intentioned friend or family member, will broach the topic subtly. Although the person is probably well aware they are fat, especially if they have gained so much weight that someone has decided an intervention is required, this seems like a moral action, because they are attempting to action in that person's best interests. There is likely no personal enjoyment or satisfaction, indeed they will probably feel very uncomfortable and upset doing it.