Thing is, a lot of people are fat because of their relationship with food. You've said how amazing you are at nutrition and fitness, but none of that matters if someone isn't in the right mental place to listen.
I eat when I'm happy. I eat when I'm sad. I eat when I'm stressed.
I know all about nutrition and fitness, but that doesn't stop me reaching for the comfort food when I'm in a bad place mentally.
How sure are you you can help? I mean, do they really need nutritional help or emotional support?
Back to your question, I don't think a size 18 can be anything other than overweight, but a size 16 could be a healthy size on a really tall woman.
You can be curvy at a size 8, you can be curvy at a size 18. A lot of people use curvy as a polite way of saying fat, but not all fat people are curvy and not all curvy people are fat.
A good acid test of how curvy someone is whether they have a small band size and large cup size. E.g. a 40H will be fat (breasts are big, but only because so is the rest of them) whereas a 28H will be skinny but top-heavy (so disproportionately curvy).
I'm currently losing weight as a direct response to Covid, but it's not actually a healthy thing to do. Taking down my BMI should in theory reduce my risk, but the way I'm obsessing over calories really isn't healthy either. I'm acutely aware I'm at risk of developing disordered eating, but for now, the benefit of reducing my Covid risk is high enough to me to let it slide.