As a discussant, should I discuss my points with the presenter beforehand?
I would go with the second option; send the slides to the author beforehand. This will help the following:


The author would be prepared in terms of the questions and topics he is going to get the questions from. He would think on a lower level and add more clarity to his answers.
Give the author a higher confidence to defend, if there seems to be any surprise.
If he feels, he can ask you for additional clarification beforehand.