It is easy to see where these things stem from. When my nan was battling cancer 20+ years ago she was getting vitC injections based on the theory that the treatment she was having (chemo etc) was depleting her stores + no appetite/vomiting so replacing them through injections is necessary. She said she felt better after them too.
Sadly she lost her battle with cancer. However, say she surived and someone who didn't have vitC passed away then people start to take 1 +1 and getting 5. I'm sure there are benefits to many complimentary treatments when going through cancer treatment but to then make the jump of turning them into a treatment in and of themselves is clear quackery.
FYI there is a whole "block Avocado Wolfe" movement set up by a bunch of scientists calling BS. Hopefully this movement grows!