I recommend you use this: http://www.jehovahs-witness.com/6/117482/1.ashx
You can almost use this as a verbatim script, try to get her to prove their doctrine on baptism to you. It is one I never encountered questions about while in door-to-door work, and I seriously doubt she will be prepared with any answer at all for this issue. She will have to research it to answer you, be prepared to let her. She will find there aren't any Scriptures to support these steps.
If she researches it out, when she realizes that she can't show you (and she can't show you from the Bible what isn't in the Bible) be kind and empathetic with her. It will be a shock to her system that she couldn't find the answer to such a simple thing; she may experience a range of emotional reactions, don't try to fix it, just soften the blow. Be patient, the turmoil will pass. But the fact that even the simple things aren't based on Scriptures is a big-time plus in favor of you winning her away from the organization; keep it as simple as possible.
If you need help as you go, just ask.
But, if she doesn't leave eventually don't keep indefinitely banging your head against a 10-foot-thick cement wall. If she is a true believer she may shut her eyes tightly against any other possibility, in which case she will probably not let you know that she couldn't find the Scriptures.