You should not directly attack her but rather ask her about the book and the author and what it is about. Then ask her about the views on different Sufi opinions and talk to her in the form of asking her as if you don’t know and in the form of dialogue and give her a chance to express herself and feels that is communicating with you and not listening to your lecture.

I recently had a cousin visit me and I noticed that one of the books she is reading is on Suffism. This sister has been practising for years so I don’t think it’s out of curiosity she is reading it nor is a ‘phase’ she is going through. I would like to ask, what is the best way to approach such a sensitive matter and advice her accordingly?