Welcome - and glad you're up to the task. Just so you know, it was my attempt to witness to a colleague of mine that helped me move on from the JWs. He said he didn't think he could ever accept the idea of a global flood as there was simply too much evidence to the contrary. At first I didn't believe him, but I did a lot of research on the various points about history and the flood, and came to the conclusion that he was right. After a while - it took a few years for me - it became so glaring obvious that the Bible's accounts of history were a joke, and it really shook me inside; how did I ever believe the Bible stories were not a myth in the first place? The fact that this was a neutral subject for all bible believers and not an attack on the JW religion made me feel completely unthreatened. Had he started in on the JW beliefs like dates and Russel and Rutherford or even the UN and pedophilia, I would have dismissed him as someone that just read some disgruntled websites. I never would have looked into his claims. But the Flood - hell, I could take on that objection no problem. Turns out - it was quite the problem.
So - just so you know, it has been done. I'm living proof, and it was just a simple conversation during an informal witness to my friend. He had no intention of dissuading my beliefs, he just explained why he couldn't accept the Bible's account of the flood. I followed the Watchtower's advice and listened to his objections and went and researched them so I could get back to him. Turns out - he was right. Global flood never happened. Maybe you can do the same. Just be patient, but stick to your guns. Make them prove it to you. If they can't prove it happened, then at some point in their life they will have to start to wonder - what else do the JWs (and for that matter, the Bible) have wrong?
Good luck.