I would try and show them the double standards that the Organization has. For example, the quote below from the 2003 WT magazine. I've taken the liberty of reversing it. How can a Witness think it's terrible when the Catholic Church changes it's teaches, but is "thrilled" with the "new light" when the WTS does the exact same thing?
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w03 5/1 p. 4 How Some Received Answers ***A schoolteacher in Portugal who had been educated by nuns and priests sincerely practiced her faith. When the church changed and abandoned practices that she had been taught were important, she became confused. Travel acquainted her with Oriental ways of worship, and she began to wonder whether there is one true God. How should she worship? When she asked her priest about things in the Bible, her questions were brushed aside, and she was disappointed.
The Catholic church had circulated a pamphlet in this schoolteacher’s city warning parishioners not to converse with Jehovah’s Witnesses. But her questions persisted. One day when the Witnesses called at her door, she listened and showed interest in what she heard. It was the first time she had talked with them.
Revised version:
A schoolteacher in Portugal who had been educated by Jehovah's Witnesses sincerely practiced her faith. When the Watchtower Society changed and abandoned practices that she had been taught were important, she became confused. Travel acquainted her with Oriental ways of worship, and she began to wonder whether there is one true God. How should she worship? When she asked her local elders about things in the Bible, her questions were brushed aside, and she was disappointed.
The Watchtower Bible & Tract Society had circulated several magazines in this schoolteacher’s city warning their Rank and File members not to converse with other religions. But her questions persisted. One day when the Mormons called at her door, she listened and showed interest in what she heard. It was the first time she had talked with them.