If they really believe that that's the best thing for cancer patients to do, then there's a very real chance that they will do that

Thinking that it's possible for some people to cure cancer via thought does in no way imply that all people who try succeed in that way. The traditional response of a person with a cancer diagnosis is to all things that promise help. Additional beliefs are needed to advice people against mainstream interventions.

Telling the person with cancer to think happy thoughts isn't harmful. It can have positive placebo effects.