Any advise or recommendations you want to give should not be silenced just because your are new or he is the GM etc. If you honestly are concerned about the way someone is playing and think it is a detriment to the guild then it needs to be brought up in a mature way. There is no reason to be held back by a team member just because they didn't ask you for help in playing better. It is your responsibility to look after yourself, and your other team members. Especially if you are hearing complaints.
One method to use is to ask the person questions as to why they have done things the way they do. Don't just tell them what to do, find out why they do what they do and go from there. You may find them willing to learn, you may find yourself to be ignorant and learning something, or you may find them(or yourself) to be a total prick. Regardless, you now have information to work with to decide what to do next.
I am little bit confused though. Are you saying he has problems with threat generation, and yet want to recommend that he try and get more avoidance? Or were you presenting two different issues, both threat and avoidance?
Avoidance will hurt threat generation, not help it. His spell damage is about on par with beginning of MH/BT content and he does have somewhat low avoidance. His low avoidance should increase threat from holy shield and spiritual attunement. When taking both his spell damage and increased threat from low avoidance into account, I strongly suspect there may be an issue with his rotation/mana management if dps is getting threat capped often.
That having been said I do see some odd gear choices, and there is room for improvement. He could, and should imo swap some of his tanking gear. Specifically his neckpiece and the ZA ring he has should be swapped with something that has defense rating, even if the stamina is a little bit lower. Swapping out a stamina trinket for the hydross defense trinket if he has it would be another option. Upping his defense would allow him to swap his chest/legs for the 2.3 paladin chest and 2.4 paladin legs, which imo he would be better off wearing in many situations.
This will lower his expertise, but honestly it seems he has gone out of his way to stack a stat that is not as beneficial to us as it is warriors. There may be a mindset issue where he does not understand expertise is a luxury for us, not a requirement. Echoing what others have said, he does seem to favor warrior designed gear a bit much for my tastes. Nothing wrong with wearing a piece designed for them, but enough of it and you will start to see a problem.
Now about his gear choices and how they affect healer strain etc. He does appear to favor stam a bit much and may not be aware that he has exceeded the effective health requirements for content above his level already. He may not be aware of how various fights can favor effective health or avoidance more than the other. Only way to know is to ask, and work from there.
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Zeal without wisdom is folly