Losingit,
The ways in which sociologists have defined cults have shifted somewhat over time. Sociologists began studying cults in the 1930s. Typically the sociolgical definition of cult has differed from the popular or "pop" definition of cult that you would see in Hassan's book.
You wrote: "I could not believe that someone who has this type of educational background conducting this type of research does not grasp the true meaning of "cults." I think he thought that i was speaking from a position of ignorance or lack of experience, but that's not so."
There is no such thing as one "true meaning" of cults. It's a social construct, and the definitions that are agreed upon by different groups differ across those groups. What defines cult from the sociological perspective isn't necessarily what you think a cult is.
A potential productive conversation would be for you to ask your friend to tell you about the history of sociological study into cults, how sociologists define the term cult, how this differs from "pop" definitions of cult, etc. You could then share what you've read, how you define it, etc. It would be an interesting conversation.
To say that you can't believe that someone with his education doesn't understand the term (as you do), tells me very little about what he knows, but demonstrates what you don't know (e.g. lack of historical and disciplinary knowledge in the sociology of cults, academic research, etc.). That's entirely forgivable but the way you wrote it comes across as condescending.
There are some interesting and fun conversations the two of you could have.