Well gemming/enchanting wise there's not TOO much wrong. He's activated his meta in the wrong place for starters though. that 10dodge/15stam gem should be in his helm, not his neck. 275 HP to chest is also far better than 10 stats, however I doubt either of these are the reason he's dying.
The real reason he is dying is due to very poor gearing decisions. Mainly due to lack of armor, at his gear levels he should not be below 34k armor, the gear he has picked both causes him to have less EHP and makes the healers work a little harder because he's taking the "convient" pieces over the armor pieces. I.E. it looks like he got some drops and was like "oh well this is an ilevel 264 belt I'll just take this over the frost emblem stuff."
Belt: The badge belt is SOOOOOOOOOOOOOOOOOOOOOO much better. It's pretty much BiS, he needs to get it ASAP.
Legs: This is the biggest one, he currenty has the weakest legs equipped that are 264 or higher. He needs to pick up Pillars of Might real bad. They are the EHP BiS and have the most armor of any leg piece in the game. The only other legs that come close are 277 Legguards of Lost Hope and those still have less EHP, just a bunch more stam but a lot less armor.
But this breaks his 4 pc!
This is why the 4pc (if you chose to go for it at 264 levels, but since you're starting hardmodes he should be getting it because the 4pc is awesome for HMs/277 level stuff) is generally taken with chest/helm/shoulders/gloves. The T10 chest got buffed and they took off the defense (which he's way over crit cap and can easily equip the chest and pillars without dipping below 536 [since he has the resil shoulder enchant]) and added a bunch of armor. It's close to the cata chest in EHP at 264 and is BiS at the 277 level.
Rings: Jug's band is bad. I would use clutch of fortification, H-Band of the Twink Valks, or the Ony25 ring over Jug's band any day hands down.
Cloak: NEEDS THE FROST EMBLEM CLOAK. It is leagues better than what he has, I'm pretty certain it's BiS.
Other gear he should pick up: devium's ring and the grimskull boots. When he drops the T10 pants for pillars he's gonna lose a large chunk of his expertise but as long as his threat is fine (which is far more dependant upon spec/rotation than gemming/gearing) then you don't need to worry about it. Grimskull boots are very good and will make up any expertise lost to acceptable levels.
Umm... that's all I can think of for now, those changes should net him ~5k Armor without much loss of stam and that will help him survive way more.
Remember folks, EHP isn't just Stam, it's a function of Armor and Stamina.
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