AK is doable in AF. It was before being nerfed so to more reason now. If he was putting out so low DPS maybe he's only using ice skills? Maybe he levelled through fates and doesn't know his class?
If it was me in that situation, first of all I would have tried to understand what was wrong and explain him the fight since he was evidently new, if it was the case maybe try to explain him how to use skills. This assuming he was responsive and willing to learn.

I once had a similar situation, except I queued with two friends so the BLM outputting no dps (notice how it's always black mages?) was the only DFed member. After wiping very stupidly twice times on first AK boss, we checked his gear and it was decent, so gear wasn't the problem, but it was all crafted stuff, so he might as well never have step foot in a dungeon before. He wasn't receptive to advice, so in the end we just said "sorry, this can't work", left the duty and queued for a new one. When you see you don't have the numbers to beat first boss, there's no chance to complete the res of the run. Especially in that dungeon, good luck beating DW if you can't beat the psyflayer.