Man,this one struck a personal note - I feel sorry for this girl. You can just see the conditioning here, and having been there myself (I remember once having a go at my mum for judging me with portion sizes because she did me the same amount of pasta as her - she was just giving me a healthy portion and I was so self-hating and unhappy I had a similar reaction) I can remember this misery all too well. This is one of the things that people who have never had an unhealthy relationship with food (or anything else that takes you over in this way) find hard to understand - it's an addiction, and just like quitting any addiction you have to change your mentality in order to change your habits. That means looking into the mess that is yourself and recognizing that you ARE in trouble and DO need help... and if that were easy we wouldn't have so many people in this world who can't break their self abusive cycle and just pass it on to others.
Aside from the mental side - she's probably cranky from sugar and fat withdrawal, and withdrawal sucks. Addicts get irrational when you separate them from the proverbial bottle, and without their usual crutch they can lash out. She's probably not used to her blood sugar being normal. Effectively she has been on a diet, and eating less can make you feel a bit odd at first, especially if don't realize how much of an effect food has on your mental state when you're used to using it to feel good.
I ended up in a similar situation (although in my case I've wholeheartedly adopted my partner's low carb diet and now cook things that he likes and have now lost 40lb and counting), and I did notice that I had some patches where I maaaybe wasn't so sane. My bf, who has the patience of a fricking saint, suggested that it was possible that I was experiencing some hormonal changes because of the weight loss on top of everything else. Fortunately now that I've removed most of my fat suit I'm feeling a lot more balanced, but there's definitely an element of 'feeling a bit crazy' when you first start restricting calories that affects some people.
Also I agree so much with the top few comments! I think some people are just never properly educated about food, and the people who should have taught them better (like their parents) are steeped in this hereditary fat logic. Realizing that it's all bullshit, that if you'd been raised the way your boyfriend has - i.e. with proper nutrition, maybe you wouldn't have spent most of your life thinking that you were powerless and unlucky, and maybe you'd have had a better life. Even now that she's seen that there's more to being healthy than just dumb luck, she's going back to what is essentially an abusive community with a vested interested in making sure she stays on their side of the BMI line and doesn't demonstrate that they aren't genetically challenged.
I truly hope this girl makes it. I hope her boyfriend has the patience to see what she's struggling with and help her change the way he looks at herself.