I'd like to throw in my support for what DJ said to og.
It was due to becoming born again that I came to question the JW's.
At the time I was happily minding my own business as a dogmatic JW Elder, on the rise to becoming a C.O..
I'm now happy to attend nigh on any denomination, but usually attend my local "Free Church of Scotland". Becoming born again was an event, not a religion (John.3:3-8).
I've spoken scores of times to the kind of event that is being addressed here.
I tend to keep things very simple, and ask that they remember and show the love of Christ, in their dealings with JW's.
If they can focus on their personal experience of meeting with Christ, it has a powerful impact. It's kinda hard to argue with someones personal testimony, especially when the fundamentals of their conversion are warranted from scripture. This also causes confusion, in the JW mind, as they can see the sincerity, and recognise some of the terms used.
Keeping the conversation Christ-centered, and not allowing it to be steered away by the JW, is the best advice.
Warn them about being sucked down the rabbit hole of discussing the Trinity, but sticking to the deity of Christ, may also help.
They really do need to be warned about having their conversation dragged into JW Lion's den topics. They need to be firm on keeping the subject singular, to be effective, and Christ is the best subject.
You could add that they have the benefit of JW's believing in the inerrancy of scripture, Genesis to Revelation. Also that JW's believe Jesus to be the Christ / Messiah.
Helping them to see what JW's hold in common (I know it's little) with mainstream Christianity, may help them to adopt a caring attitude for the misguided.
It's pretty straighforward to then show where the JW's definitions of common theological words, falls short.
You also want to stress very strongly the danger of them having their faith subverted by what appears to be strong reasoned arguments supported by scripture.
To recap: Jesus, Personal Testimony, Jesus, common ground, Jesus, firmly stick to a topic, oh, and did I mention Jesus?
The reason? Born again Christians know heaps about Jesus, from personal experience. Jw's do not.