I don’t think you have any experience with eating disorders or women in general.

That is nothing more than your assumption and you assume wrong. I will not tell you details about my private life, because it´s not your concern, just this much. I have several years of painful first hand experience of living with a women who suffers from severe eating disorders, so I intuitively recognize certain patters the moment I see them.

Someone who used to be skinny gaining a bit of weight and feeling fat is not “behavioural patterns connected to anorexia/ bulimia”.

That is not what OP described though. You are diminishing the facts. OP wrote:

She was underweight before because she hardly ate anything. She’s way better now.

So let´s believe OP and consider that she was not just skinny but in fact underweight, which is a very important and significant difference when talking about eating disorders. Furthermore, consider that the reason OP states for her being underweight before is that she used to "hardly eat anything" which clearly points to anorexia. Now combine being underweight and hardly eating anything earlier, with the fact that not being underweight anymore makes her feel "fat", which points to distorted self perception and misguided body awareness.

And even if it was, how does “this is clearly freaking you out, let’s ignore it and do this instead” supposed to help?

By avoiding a discussion that will only do harm to someone with an eating disorder, independently of what you say to her. Did you ever have a discussion like that with someone who suffers from an eating disorder? I had them hundreds of times. There where in fact phases when I had such discussions on a daily basis and trust me, you can only loose there. Btw, I did not come up with that sentence myself. It´s from a website that gives professional tips about how to deal with people who suffer from eating disorders. I wish I would have known about it back then, when I would have needed it. Instead I fell into the trap over and over again and I wanted to help OP to not make that mistake.

I’ll assume you have the best of intentions in mind

You assume correctly.

if that’s the case you really shouldn’t be giving advice here because everything you’ve said so far is harmful.

I honestly do not understand how, because my intention was the exact opposite of doing harm. Please elaborate.