I don't know if logic will change something in her belief system. I also explained once all the logical flaws of the flood story with my wife. She says that probably the flood was local and not global. But who cares 'We will find out in the new system.'
It's however a good way to do it with questions.
On this thread we discussed about the flood too.http://www.jehovahs-witness.net/watchtower/bible/268118/1/Dr-Hans-Kristian-Kotlar-supports-creation