Hi
You may have some difficulty trying to convince a JW that the parable about a "wise and faithful servant" is not a prophecy as the WTS insists. So I would approach it from this angle.
Ask the JW these two questions.
Why has the WTS been uncertain of who the "slave" actually is, requiring that they redefine the identity of the "slave" numerous times since first adopting this doctrine. There is no biblical example of a servant/prophet of God who was unsure of his own identity over a 100 year period. That notion is utterly ludicrous. The WTS has continually reinvented the identity of the slave since day one. God is not the author of such confusion.
Next would be to examine the fruitage of this supposed "chosen servant of God". Virtually every time prophecy/prediction that this "slave" has made has failed. Can they site a similar scenario in the bible with true prophets of God who had a 100 year track record of failed predictions. I think not.
If your friend does not respond to this information, he may never see the light, but at least it is worth trying.
FWFranz