Arcane does suck at the moment its a well known fact at this point in time. The best advice I can offer is following some of the tips above (not the respeccing ones) and for him to wait it out and see what 4.0.6 brings us. Ghostcrawler has said that arcane's damage is too low atm so now all we got to do is wait and see what changes blizz have in store for us. I suspect we will be seeing more very soon.
Arcane has two strong points: burst damage and control.
Arcane dps easily triples in burn phases, on-demand, when you want it. Sure it will be lower that fire overall, but it will be way higher than fire when it's most needed.
60% slow is invaluable for kiting/controlling adds. Try handling adds on Erudax in Grim Batol as fire, having no means of controlling them in your party. Arcane mage can slow them both (one with slow, other with arcane blast slowing effect). It's just a dungeon, i know, but it illustrates my point well. After we downed him our tank said something like "i wonder if it would be doable at all if you were fire?".
sorry, you missed my point :P however this person has NOT missed and i';m glad for that
my point was that we cannot go only by a hard cold number like stateofdps or recount for that matter :P
and as someone else has said , we(arcane) can provide insane advantages as to the control we have on our burst, i decide when i need the most damage, i choose when to use all of it, and as single target i have yet to meet any mage to out dps me :P not bragging or anything, just a notice
oh and wo ignore my armory, account frozen ( ) logged out in "dungeon" gear ( i.e. lower hit->more dmg )
While I don't doubt that Arcane could have its place in the raid environment other than being the buff-bitch, it has glaringly obvious issues the other two specs don't.and as someone else has said , we(arcane) can provide insane advantages as to the control we have on our burst, i decide when i need the most damage, i choose when to use all of it, and as single target i have yet to meet any mage to out dps me :P not bragging or anything, just a notice
It has little to no mobility at all, which is absolutely pathetic since it used to be one of the best mobile specs when Wrath started out. Granted, it was over two years ago and I won't disagree the coefficients for spell weren't ridiculously out of whack, but the fact it's gone the way of Elemental and become a turret-y spec in an expansion where movement is heavy is enough to make me stay the hell away for a good long time.
It also has fairly awful AoE, especially when compared to fire. The single fact that you have to run in and start spamming in order to do decent AoE is ridiculous. The only other ranged class that had that issue was Elemental, and again, that was rectified with Earthquake (relatively). Nevermind the actual liability it is to yourself depending what the trash/add packs actually do mechanics-wise. I'd rather see it follow the Living Bomb idea where you cast it on a target and they explode for Arcane damage, which damages everyone around them. There's other ways they could work it; it's simply really back-ass-wards when the expansion calls for you to be proactive and avoid as much damage as possible while saying 'here, run in and spam this spell, try not to die, k?'.
On topic and TLDR:
Unless you have NOBODY brining the 3% damage buff and you absolutely are desiring that, have your friend go frost or fire. Being Arcane is a relative raid liability as well as dps liability simply based on how it's designed. If he's stuck on it, send him over to EJ, or there's probably a thread here, even.
Sadly, that isn't true:Arcane mage can slow them both (one with slow, other with arcane blast slowing effect)
Nether Vortex- Gives your Arcane Blast spell a 100% chance to apply the Slow spell to any target it damages if no target is currently affected by Slow.
Though, Frost Nova, Frostbolt, Cone of Cold, and Ring of Frost are still usable.