No one wants to be thought a fool. Much of the success of these con artists is directly attributable to a dirth of support for the terminally ill patient. When the support is there, the con men have a more difficulty getting a foothold. There are, unfortunately, a number of physicians who, when there is no real Hope, reject their patient. If the patient has a supportive family, that helps a lot.
Some people have no support at all and they are at real risk of falling for the con man's pitch.
Con artists are fairly easy to recognize. They are enthusiastic about their product and act kinder and more considerate than anyone has a right to expect of a stranger. They are eager to condemn the medical profession. They will also tell you that people are trying to suppress their discoveries. They have a logical explanation for why the treatment will work.