I find using examples from other religions is always a good starting point. I made a video a little while back about if a follower of Harold Camping ever discussed beliefs with a Jehovah's Witness, I'm sure you can figure out how to turn this into a conversation with your friend. Just make sure they're agreeing with you as you go.
"Have you watched the news about Harold Camping... wouldn't you agree he's a false prophet? And the way he just changed his story at the last minute after his prediction proved false to it happening invisilby, could you ever believe someone who did that? Isn't that the same thing the JWs did in 1914? I'm having trouble understanding why it's OK for JWs but not for Camping to do the same thing?"
Here's a link to the video:
http://youtu.be/OM2rfOYnK_M