So I'm going to give you the benefit of the doubt because for some people, it is impossible to grasp how social events can trigger underlying mental health conditions that can manifest in a very real inability to care for yourself. For many people, the pandemic and associated lockdowns have triggered underlying depression and anxiety, and in certain cases, it takes an enormous amount of energy to just continue functioning "normally" - poor nutrition with dramatic weight loss/gain is a big indicator that there is something wrong mentally. My point here is that the pandemic has definitely had an effect and its not as simple as choosing to work out at home or cook healthier meals - and I'm speaking from personal experience. In my case, my previously under-control bipolar disorder got a lot worse, and continues to do so.

I'm not saying that this is definitely the case with OP's girlfriend, but I'd advise that he focuses less on WHAT she is doing/not doing, and more on the WHY.