I'd start off by saying something along the lines of: "I used to be a witness..." That's make them perk up and think they'd met a fellow witness they can get back to meetings single handedly. 
Then maybe "I want to warn you that everything isn't quite right once you look into it, please, please do your research and don't be scared to ask questions about stuff. I was too scared for years, and it made me unhappy for many years until I finally broke free"  "Trust me, I know everything you've been taught, I was a regular pioneer for 3 years and preached everything you preached. But once you look into it, it just doesn't make sense"

Just be very careful about the wording you use.  I was still 'in' as recently as six months ago, so it's still pretty fresh in my mind.  Terms like "used to be a witness", or "until I finally broke free" would just scream apostate to me.  Maybe say you were raised a witness, but that you haven't been to a meeting for a long time.  And be careful about saying stuff like 'broke free' - active witnesses really don't understand how difficult it is to leave, so if you make any comment about 'escaping' it will sound wrong to them.

When I first started doing my research, I tended to oscillate.  I'd do a little research, find some uncomfortable truths, then go back to a meeting and be all 'i love jehovah, i love jehovah' thinking maybe if I delude myself enough it will be true again.  And gradually I'd get the balls to do a little more research, and back and forth like that till I eventually came to terms with it.

I've put some thought into it myself... of course, depending on if I was in an area where people would recognize me, I would just play dumb.  The reason I accepted jdubism as a kid was because I had no critical thinking skills to speak of.  As an adult, at least now I know the right questions to ask when presented with new information.  

So, were it me, I'd ask general questions about what they believe, and ask if they have any evidence to back up those claims.  If there's no evidence to support their teachings, then how is it any better from any other belief we could make up?  The common jw response is 'once you establish some of what we say is true, then everything else must be true'.  That's simple to refute - 1+1 is 2, 2+2 is 4, I was abducted by aliens last night, I'm married to Elvis and the Loch Ness monster is living in my swimming pool.  You could say a million true things in a row and then start saying nothing but lies without any difficulty whatsoever.  

And to make it a little more poignant, I was raised by my exjw mom, and she was the main reason I became so active with the jws.  If she had just taught me neutral critical thinking skills and voiced her respect for whatever decision I made, I probably would have left before I became a teenager.  Instead she was very adamant that I don't have anything to do with the witnesses, and that was the main reason I became a witness.  So just be careful about speaking too strongly against the org, because it will always backfire unless someone's about ready to leave anyway.