Cherryade, I'm sorry to hear about your friend. This is a frustrating and upsetting issue for you to witness.
I had the same experience with my mother (who has Alzheimer's) regarding food. She went through a period where she didn't eat and lost a lot of weight, and became malnourished. She also went through a period where she ate everything in sight and gained weight.
Part of the problem, for my mother, was that she was unable to cook herself a meal, plan a meal, shop for the correct ingredients, et cetera--an executive function problem. The loss or diminishing of executive function can happen, and the PWD (person with dementia) can be unaware that it's happening, or in fact, because of the disease process, unable to understand there is a problem (that's a whole other issue, anosognosia).
Another part of the problem was that she was no longer feeling hunger and thirst in the usual way, so she would refuse offers of food, as she wasn't getting hunger signals from her body. She would refuse food up to and after the point where it was placed in front of her, in fact. Then she would often say that she still wasn't hungry, but would have a bite "to keep [me] company." I also learned I had to sit directly across the table from her and keep eating and drinking until her plate was empty, as she would eat the entire meal, but only as long as I was eating. She needed both the social setting, and the visual cues, to eat and drink.
And another piece of it is that the dementia has changed how things taste. Either sweet foods are more appealing, or the body craves sugar and carbohydrates, or bitter things taste more bitter, or all of the above. I'm not sure what the mechanism is, or what is happening in the brain.
What eventually worked for my mother was moving her into a care home, where she is taken to meals three times a day and she mostly eats what they give her, and her medication (including vitamins) are given to her as well. Luckily they were able to treat the vitamin deficiencies in time, before damage was done.
You might find some information here that is helpful:
https://www.alzheimers.org.uk/site/scripts/documents_info.php?documentID=149
The only practical ideas I have, besides someone preparing healthy meals three times a day and sitting down to eat with your friend, are some ideas I've seen here on TP.
Some PWDs go to a lunch club or adult/dementia day care one or more days during the week, where at least one meal is provided.
If Meals on Wheels is available in her area, that could be an option.
Prepared (frozen) meals from the grocery store or a delivery service like Wiltshire Farm Foods, although this might also need a volunteer or paid carer to come in several times daily, heat them up, and serve them. There can also be problems with the PWD not eating them after the carer has left, or emptying them out of the freezer, but you don't know until you try.
A carer can also prepare and leave some other items to be snacked on during the day, like sandwiches or fruit and cheese, veggies, et cetera. Again, that could work great or not.
Especially if there are any other medical issues (like diabetes or a thyroid issiue or something) going on, I'd make sure the GP is made aware, so they can do any appropriate blood tests. Even though someone's doctor likely won't/can't talk to you, because of privacy issues, they can listen to anything you tell them.
Sorry I haven't better advice. Best wishes to you both.