I would try a different tack and ask her if she wasn't a witness what would she see herself doing and what ambitions would she have fulfilled? Get her to think about what the religion does to people and the impact it has on their lives.
Ask her why she never pursued any of her ambitions? Why can people of other faiths maintain their beliefs whilst at the same time using their skills to the full?
If the ministry is so important why do most witnesses spend less then 10hrs a month on the ministry and still do nothing to improve their lives? If the majority of witnesses do not fully commit to their beliefs why should I?