Another suggestion, taken from my own personal experience.
Many books have been written on this subject, and they articulate their arguments much better than somebody who is just starting to "counter witnesses" to their loved ones.
My goal was to get Ray Franz's 'Crisis of Conscience' into my wifes hands and let her read it. I knew that it could do a much better job than what I could do. The problem was fear. I slowly worked on the 'fear of apostates' (simliar to what you have tried) over a peroid of time. When I felt my wife may be ready, I introduced her to the book.
She didn't want to read it at first. It sat on a bookshelf in our bedroom for a few days, completely untouched. Eventually she picked it up and started reading.
As long as you are able to develop an atmosphere of trust and acceptance, the other person may just explore new things. But don't move too fast. I think I introduced COC about 4-6 months after I had actually read it.