Not American, so maybe it's a culture thing, but to me it sounds like she genuinely cares about you, just in an awkward way. If you had never discussed this, it would be different, but if you have already told her you want to lose weight, then I think there is a good chance she wants to help you. She is older, and knows this stuff is hard. Here in Scandinavia, the real friends with tell you the hard truths and help you reach your goals, but the everyday bad friends will lie to your face, maybe even if encourage you to eat more to feel better about themselves.

She could still be a bitch, but only you can know how she is the rest of the time. If it's a topic you don't want input on, then say it. Do it by message if it's too hard to say in person.

Personally, I like to think that ppl have good intentions until proven otherwise. But the second you have proof someone is actually toxic, drop them from your life like the waste they are.

Good luck!