For Advert Harm on Thok I use it when I am not tanking the boss. It can actually help out as more screams go out but he is gonna have to be aware that they will need the haste to purify the damage that he is taking. It is a good CD for that fight in my honest opinion.
I would say that he needs more haste because the amount of damage that he is taking would be to hard with his level of haste. That level of haste was ok for the 4 set tier 15. But with the new tier he needs to have enough to purify to get the most out of the bonus.
Using Breath of Fire is only on if he can keep up Shuffle. I honestly never use Breath of Fire unless it is on the adds from Garrosh. I don't think that it is a great DPS boost at all and so I would suggest to stop using it all together and just do BoK.
If he is getting lower on health he needs to be able to use Expel Harm more. I kinda use it when I see my health lower then 50% health just because it gives the healer time to react.
This spec is one of the hardest to learn because you have to keep the buffs up but it is probably the most rewarding as well. So good luck and if you need anything else let us know