I've never heard of a Witness coverting to another faith unless they themselves have found out (for themselves) that doctrines are false.
You need to realise they will have all the answers and NOT ALL witnesses have doubts, some do, others blindly follow and believe it is the truth no matter what - you will not be able to reason with these ones.
They are prepared for spiritual warfare. You may mean well but I doubt you would get much in - they really would ""win back the conversation"" and turn it to their advantage.