One of my friends believes that eating an extremely healthy diet will cure cancer. can it?

It may help: In general no. However, in some diseases (brca, colon ca) diet may decrease the risk of recurrence. So, i would suggest that a health lifestyle including diet can contribute to cure when used adjunctively with other treatments. In some cancers diet may have no improvement at all. Cancer is common (1/3 women and 1/2 men lifetime risk). Even people that do everything "right" can and do get cancer.

Help, yes. Cure, no.: There is no doubt that what we eat can contribute to the development of cancer. Unfortunately, once cancer develops, it cannot be treated exclusively by dietary changes. However, eating well can certainly compliment traditional therapies and may prevent the cancer from returning.

No: Diet has been associated with cancer development, contributing to approximately 30% of cases. However, there is no scientific data that cancer can be cured with any type of diet, including "an extremely healthy" diet.

No: There is no evidence that diet will cure a casncer once it has developed.

No: It's human nature to want to believe lies that give false hope, especially when there's talk of "natural healing", "wicked physicians suppressing the cure", etc.