dgp, it was someone like you that I worked with that helped to open my eyes to the big lie. This was before the days of the internet and his knowledge came from being a curious intelligent person.
Your knowledge is a valuable thing. Firstly, the fact that you understand the religion should open your colleague to discussing it with you. It sort of makes you seem like a potentially "interested one", which keeps communication open.
My advice to you is to translate all beliefs and activities into normal language, rather than to quote the literature extensively. For example, ask wide-eyed... "Does your group teach that everybody on earth is soon to be killed, except those in your group who will then be biologically changed so that they don't age, ever?"
The response will be something like "We can't say who will be saved", but the seeds of absurdity will be planted.
"Does your group believe that it is acceptable to celebrate the anniversary of a marriage, but any member who celebrates the anniversary of a birth should be shunned, excommunicated, never again spoken to?"
There are so many examples. Turn it from cult-language to normal language.