I dare not step inside that accursed tower myself, of course.
While I cannot tell you exactly where your comrade is being held, I offer a gift to assist you.
Use it inside Torghast. It should take you somewhere near your missing ally. Baine Bloodhoof was his name, yes?
Either that, or you'll end up trapped within the tower's ever-shifting halls for all eternity.
But those are the risks one must take, I suppose. Best of luck!