Robert Young, 63, has been charged with theft and "treating the sick without a certificate" in California.
However, GP Avril Campbell and retired gastroenterologist Booth Danesh, who are the parents of the singer and West End stage actor, appeared in a video praising Mr Young who claims to be able to cure cancer with avocado juice.
Dr Campbell, who works at Glasgow's Beatson cancer centre, was recovering from breast cancer when she visited Mr Young's alternative retreat outside San Diego. However, a charitable trust that aims to help people make sense of scientific and medical research said yesterday that doctors were vulnerable to being targeted to endorse controversial treatments.
Emily Jesper, assistant director of Sense about Science, whilst not commenting on Mr Young's case said: "When living with a debilitating condition, particularly if there is no cure or current treatments aren't providing relief - there is a potential market in false hope.
"We see it time and again. People with chronic diseases offered unlicensed stem cell treatments, and expensive unorthodox cancer 'cures'.
"What these situations have in common is the layer of respectability added by involvement of 'international experts'.
"Medics and scientists need to make sure they don't unwittingly become part of this problem. But rather, give critical scrutiny of extraordinary claims."
Britain's leading academic critic of alternative medicine, Edzard Ernst, said: "Cancer patients are understandably desperate and thus all too often victims of charlatans who promise a side-effect free cure.
"However, an alternative cancer cure does not and will never exist. If a therapy looks vaguely promising, it will be tested and, if effective, adopted by conventional oncology."