Firstly, it looks like he's using army of the dead on pull? That should be saved for when bloodlust is used. His two trinkets are terrible, 840 crit stat sticks are better than those. His talents are fine. In general his itemization is quite poor, crit is his best stat, yet he has almost none of it. For example, i'm sitting at ~35% crit and a little over 20% haste. The mark of the claw enchant is terrible, satyr is the best one by far, with armies close behind. I would suggest picking up some crit haste crafted rings to help fix his stat problems.
Now in regards to the boss fight itself. He needs to learn how to effectively use his/her cooldowns correctly. In this case, you can see that you will only get two uses of gargoyle, therefore instead of using gargoyle on the 3 minute mark, he should be saving it for bloodlust at that 30% mark.
Another major point of issue, is the usage of soul reaper. Quite often he's either straight out forgetting to use it for a whole minute or gets distracted. It's a 45 second cooldown, and our strongest one. It has to be used on cooldown. You should KNOW it is about to come up, and therefore prepare with wounds / runes beforehand.
One more important factor for increasing damage on this fight, especially on the nightmare images is proper pet micro. As you can see in a bunch of those attempts, his pet is doing very little to the add and boss during the time those adds are up. This can be remedied somewhat through manual pet micro. His dark transformation usage isn't that bad either which is good. Otherwise not too bad, it will take him a little bit to practise all of this at once though so don't be too hard on him, especially the pet micro.
Edit: also using 375 crit food instead of 300 haste would help too
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No those two talents are superior, given there is cleave.
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Mastery isnt a bad stat, its just terrible for ST, but if he had the bracers it would be a different story, crit ends up in the dumps and then haste is king with mastery secondary.