Mastamage posted a video of him doing Garrosh Heroic as arcane on the video page. I'd suggest linking your guildie to the video, it gives a very clear strategy.
http://www.youtube.com/watch?v=xoTpu09LxRU
..and while heroic does add some mechanics, they carry over well to normal garrosh.
You, as a guild/raid team, will have to adapt to the limitations of him as arcane. In the video I linked, the mage is in melee as much as possible, so you have to treat your guildie basically as another melee (meaning, he cannot be on the siegeworkers either).
If this is not an acceptable strategy for your group, he will have to switch to fire - end of story.
He also needs to have all expertise/mastery (keen) gems, not int/mastery (artful), right now he has 6 keen and 7 artful, it should be 13 keen.