Ask the question - let them answer. And then leave it alone.
I think this is key. Rather than coming at them with information that could shock them back into their safe JW rabbit hole, I think it's best to casually ask them to explain their own beliefs to you.
Once I stopped going to meetings myself, I used to ask my wife to remind me what the scriptural reasoning was behind a certain basic JW doctrines. In the process of her telling me the answer, she'd see how flawed the reasonings were. I didn't have to do anything, just let her explain her own beliefs.
Sometimes I'd ask her what her take was on something I wondered about. For example:
Me: I was thinking about how God sent his angel in to kill off all of the firstborn babies in Egypt because Pharaoh wouldn't let the Israelites go.
Her: Yeah, what about it?
Me: I was wondering why he had the Israelites organize an army and do battle for him on many occasions when he could have simply sent in his angels to get rid of his enemies, such as the Cananites. Especially since one of the 10 commandments he gave to them specifically said, "Thou shall not kill" ?
I'd leave it at that, not make her feel obligated to answer or defend anything. I'd just let her think about it.
I started to notice that when I attended a Sunday meeting on occasion, she'd look up the scriptures that were sighted in the talk but would begin reading the surrounding verses and would notice that the speaker was applying the scripture out of context.
Finally one day, she went by herself to the District Assembly but came home at the intermission disgusted by what she was hearing them say about "higher education". She hasn't been to a meeting or Assembly in about 7 years now. She quit on her own...cold turkey and unlike me, doesn't feel the need to discuss it with her JW family, hasn't felt the need to visit a site like this and couldn't care less what's going on with the JW religion now.