Steve2 wrote: " If your friend is a researcher in the field of Sociology, he should already be reading widely on his chosen topic, including the book you mention. It would be unusual if he has not heard of Hassan's work."
I do not agree that a sociologist studying cults would have to be familiar with Hassan. Hassan is a mental health provider and writer of self help books. This is in no way negating the important work he has done. His work would not necessarily be on the radar of an research sociologist studying cults. He hasn't published any academic pieces that I am aware of.
I am an academic, researcher and writer working at the intersections of psychology and sociology with degrees in both fields. I do not pay any attention to pop psych and self help books in areas that relate to what I study. That said, I may come across them accidentally and might poke around out of curiosity. But they wouldn't be books I'd be reading, nor would I cite them unless I were somehow studying self help movements.