@Dogingarden I think your friend is being undiplomatic offering unwanted criticisms but maybe it does come from a place of concern and ignorance about cancer and the effects of chemo.
Being realistic, if you are getting limited food than the healthier it is the better, so I see her point, but what she needs to understand is that this isn't a fad, an intentional bad choice, its survival mode, and normal dietary rules go out of the window in favour of any sustenance you can get down your throat (and keep it there).
This is in line with you consultants advice, which will be born out of many years of seeing patients going through this exact scenario. His guidance, no, medical prescription is to do what you are doing. Your weight is extremely low for your height.
Nonetheless, in her head, she probably (wrongly) perceives she is trying to help, as annoying as it may be,
I would be tempted to talk to her and only ask her to leave if she continues to ignore your stance. I would probably say something along the following lines:
"I need to be really clear with you. I’m 5ft 9 and only 7 stone right now. My medical team have told me outright that stopping the weight loss is critical, and the only priority is getting calories in however I can. This isn’t a lifestyle choice or a fad, it’s a serious medical condition, and I’m following the consultant’s instructions exactly. He has advised me getting something down is the most important thing.
Your comments about ‘processed crap’ were hurtful and inappropriate, especially after weeks of having no appetite. You should be grateful I’m eating anything at all. I know your advice comes from a well‑intended place, but it’s not appropriate here, and it’s making an already difficult situation harder. I'm struggling as it is with everything going on and unwarranted criticism, disguised as help, is bringing me down."
And if she still isn't getting the boundary, then add:
"I need to protect my health and my peace, so I think it’s best if you head home today and we will catch up again in the future."
I was seriously ill last year and despite being know as a food lover, I just couldn't face it and unless you have been there, its hard to explain to others. But she is obviously visiting because she cares so don't burn the bridges for the future. Clear boundaries are what is needed.
Wishing you all the best OP.