Do not give unsolicited diet advice.
Just don’t do it! Not even if you just went on a diet you think is great. Not even if you “care about the person’s health”. Not even if you’re just “doing your job”.
If a person asks you for diet advice, by all means go for it. But if they’re just trying to mind their own business and live their own lives DO NOT PUSH YOUR DIET IDEALS ON THEM.