This was the argument I used:
This was a discussion point I had with my wife. But, I think it is really better to not discuss doctrine. They cannot see the other side of an issue.
Prior to the outpouring of Jehovah's holy spirit his followers made all sorts of mistakes and had disagreements about what was correct, even the apostles (read the threads on circumcision). But, once the holy spirit was poured out that stopped. The organization claims Jehovah's holy spirit is active upon it, therefore they shouldn't have to change doctrine or make the mistakes like the Israelites made.
I wish you the best of luck.