The fact that he can use both perma affect items at once MIGHT be a bug, however damage reduction in CF is applied in a pretty consistent manner.

At the end of the day, it's just stoneform + stoneskin affect. Given what we know about those affects and how damage reduction is applied, I think folks can draw some pretty reliable conclusions about what's going on.

Obviously the only ones who know for sure are those that can examine the code, but given the circumstances combined with common knowledge, I'd find something else to troll about if I was you.