I have heard that. I'm not a nutritionist or medic, but my understanding (from having had chemo and doing a little bit of research into complementary therapies) is that super healthy eating and nutritional supplements such as anti-oxidants will not 'feed the tumour' or interfere with the effect of the chemo. They might have a slightly protective effect on the healthy cells.
I took a mushroom supplement which is supposed to help the immune system recover more quickly but had to have the dose of chemo reduced quite early on as my 'bloods' were too low for the full dose, but I have a funny immune system anyway (multiple food intolerances and chronic fatigue)
My oncologist was happy for me to take supplements but I think it varies from consultant to consultant. A nutritionist (not a dietician) would probably be happy to advise.
You might like to google the Budwig diet.
Also green tea, turmeric and reishi mushrooms. Ideally your friend should probably avoid refined sugar but Tbh if her mouth is sore and/or her appetite is fragile or her sense of taste goes awol the motivation to eat super-healthily can get squashed.
Shark cartilage is not worth bothering about btw.
I got quite paranoid about avoiding infections but everyone is different in their approach. Sometimes, 'rebelling' against being super-cautious and sensible is probably good for the immune system too!
One thing I found is that there's usually a lot of waiting around at hospitals for blood tests, appointments, prescriptions etc. There's no point getting het up about delays or hoping that everything will run to schedule so that you can do something at a fixed time. If I was sitting beside anyone who was complaining about the waiting I'd excuse myself and go and wait somewhere else! I used to 'use' the time to think about things, send texts, doze, people watch, flick through magazines etc.
But I couldn't stand reading about superwomen who had worked fulltime throughout their treatment despite being exhausted etc etc blah blah. Good for them. But I didn't feel the need to push myself for the sake of it. There were times when I made myself go out as I knew I'd feel the benefit of being safely inside again and a couple of events I was determined to go to come what may, but everyone has their own way of coping.
Also, I don't know which cancer your friend is having treatment for, but I used to get irrationally fed up of hearing about breast cancer
(mine wasn't breast cancer).
The medics won't prescribe your friend more chemo than she can cope with.