Is he actually Crush Immune without blowing cooldowns? I know paladins can do it, but they use Holy Shield, their version of Shield Block that gives a huge 30% extra chance.
Assuming a very generous 45% avoidance without BT/Hyjal gear, he needs 57.6% more mitigation stats to push Crushings off. Which amounts to... a whopping 453 block rating...
Being 15% away from Crush immune is just as useless as being 50% away. Even if he manages to push all but 5% of Crushing Blows off, when they do come, it'll result in a huge spike of his health bar, likely killing him.
Maybe he's counting on all the different debuffs that the raid can put up to reduce that 450 required rating, but that's a huge risk to take. Furthermore, he'd be useless against magic damage, which isn't that uncommon.