It sounds like she is definitely not fat, so you can be truthful and you aren't looking for a way to tactfully say she is at an unhealthy weight. What it sounds like is happening instead is a bunch of busybodies are just stirring up trouble and trying to undermine her self confidence. If it wasn't her weight, it would be her clothes or some other body part they would criticize. (for instance they'd claim she had a weird nose or ears, I had a "friend" comment on my how my knees looked weird and knobby one time. They were and are normal knees. My grandma tried to make me feel like there was something wrong with me because my breasts hadn't come in yet. I was 12. Both of these were people "looking out for me and trying to help" - they were not. They were trying to make themselves feel better at my expense.

You need to make sure she realizes these comments other people are making are not motivated in kindness, even if they are claiming they are. Try to find ways to help her see her worth and to help her ignore the bullying comments by these people.

Because of mainly my grandma, I learned to recognize when these comments were meant to be mean and to not let let them affect my self-esteem. Instead I realized they just made the person saying them look worse.

Occasionally, when they would get a comment in about something, like a big pimple, I would gray rock it and respond with, "yeah, that happens, oh well" and move on. Learning to not give them a reaction also makes it not fun for them after a while and they find other targets or shut up.

Learning to gray rock and not internalize the crap other people are flinging will help a lot. Having someone like you that she can trust to be actually kind and honest will help her reinforce to herself that the other people are just being unkind.