9 out of every 10 casts? So he only successfully interrupts 1 Pyroblast cast per Initiate? He might be trying to interrupt the Brushfire casts which, while interruptible now post-nerf, are not the priority spell to be interrupting. If he is missing any Pyroblast casts the Initiate is buffing himself with Fire It Up! which will increase the cast speed of the Initiate by 10%. So if he starts missing interrupts it will only get harder as he goes along.
Do you have any logs for us to look at?
He logged out in ret spec/gear. All I can look at is his spec. Only a couple of things to notice there. Hallowed Ground is a weak talent. Consecration has never been the threat magnet it has been accused of being. Not only that, he has glyphed for it. Better is to fill out Guarded by the Light. Take the last point over to the ret tree. Personally, I put that point in Pursuit of Justice. I also take one point out of Rule of Law and fill out Pursuit. Run speed is hard to quantify but impossible to replace. It also allows using the 50 mastery boot enchant instead of Lavawalker.
Having said that, those are small things and do not explain taking excessive damage. Without seeing gear, it could definitely be involved with that. However, it could just as likely be mechanics such as turning his back to move instead of strafing, not using cds, not being aware of mechanics, etc.