But I'd see this through the lense of assisted suicide. An addict has the right to use drugs, but if I prepare the needle I can be criminally responsible?
If a parent feeds a child like this, it is neglect? If a patient demands medication that will cause harm, should a doctor prescribe it?
I think he does have the right to eat himself to death, but a carer does not have the right to assist in that death.
A parent over feeding a minor is a very different scenario to an adult with mental capacity deciding to overfeed themselves.
Prescription medication is not a right. It is only available at the discretion of people highly trained in the field, never by demand. That gives the Dr every right (and indeed obligation) to say "no" if the drug is not appropriate for the patient. Food is available to all. There are no restrictions on it. No-one has the legal right to say "no" to another adult who wishes to eat something deemed inappropriate.
You agree that a person has a right to eat themselves to death if they so wish. As has already been mentioned earlier in this thread, people are completely misunderstanding the role/power of a Carer. A Carer is not medically trained (past a basic level). They are there, in a person's home, to help that person do whatever it is they can no longer manage for themselves. You may view cooking & serving fatty foods to an obese person who chooses to eat them as "assisting in a death". The law would consider refusing to do so as abuse of a vulnerable adult.
Carers can no more refuse to serve a certain food (and in this case, the man was shopping himself online) than they can refuse to pass a smoker their cigarettes or allow an alcoholic to purchase vodka on a supervised shopping trip.
A Carers role is to help a person to live their own life the way they choose. The way they would live were they physically able to. Not to judge. Of course you can make suggestions - after all, most Carers actually DO care (contrary to popular belief) - but you can do nothing if the person isn't interested in those suggestions.
As has already been said, this man and any like him, will have see many many HCP's over the years. If highly trained, professional people haven't managed to persuade a person to change their habits why would anybody expect the hired help to be able to?