I have to admit, there are many delicious scenes running through my mind about the angst (the woe! the torment! the betrayal!!!) that Harry would experience in confronting Snape with this atrocity. What can I say--sometimes I'm just into the angsty angst. :D
In a more serious frame of mind, I would guess Harry would react--rather badly to finding out. The bulk of it would be shock, I imagine, because Severus loves Harry so absolutely--and Harry would have proof of it too, in every conceivable manner (Veritaserum! Polygraph! Blood rites! Human sacrifice!!!). Well, maybe not that last--but Severus would insist, probably up to (but not over) the point of coercion. He will have Harry believe--and so Harry shall believe, in the end. But it would take terrible trials and many, many pages of writing to get there, and alas...I haven't the patience!