No problem-- in no particular order (buckle up, wall of text inc):
He cast shattering star 7 times. This is a massive deal. Shattering star should be used on cd and when possible lined up into an eternity surge (fire breath before surge) and then followed by both mass disintegrates. For the length of fight you had, he could have cast SS 27 times.
He missed an entire dragonrage window. He cast 4, could have gotten 5. Your fight also took a very long time which is probably not helping, considering how devastation mastery works. 8:20 is about 3 minutes slow for current gear.
He used deep breath 5 out of 11 possible times. Deep breath is a huge part of the devastation rotation now.
He missed about 10 total casts each of fire breath and eternity surge. This also means he missed 18 mass disintegrate casts. This also means that he missed well over a minute of having bombardments up on the boss. Again, a huge DPS loss.
Looking at his dragonrage windows, across four of them he only cast a total of three empower spells. That’s very bad, because casting empowers not only gives mass disintegrate and bombardments, but it also extends the duration of dragonrage each time you cast them. He should be casting each empower twice within each dragonrage window.
He is also routinely chaining mass disintegrate into regular disintegrate. What this means is that with one tick remaining on mass dis, he’s pressing it again and losing that empowered tick. This is the right way to cast normal disintegrates, but you always want to either stack two mass d’s with your back to back empowers and let the final one complete its channel before starting a normal disintegrate.
He’s wasting essence burst and burnout procs.
He’s literally never using his defensives (obsidian scales and renewing blaze).
I will assume the lack of food, flask, mana oil, and are due to the start of the season and that he’ll start using them soon.
If this person is a solid player and just new to Dev, this just screams inexperience and panicking in the heat of a fight.
I’d strongly recommend he hits target dummies for while with a rotation guide up from preheat/wowhead and spends time getting muscle memory.
Although I personally think they are a crutch that can create bad habits, having him download hekili to guide his rotation and build muscle memory to start may be helpful.