I mean, speaking in terms of optimization, yes, tanks should drop tank stance as soon as possible and stick to DPSing throughout the fight; if they do their opener correctly and the OT vokeshirks them regularly, that and tank swapping should provide more than enough aggro to not require tank stance for the rest of the fight. That being said, presuming good coordination in a PUG group as opposed to a static is dangerous, and if he wasn't communicating beforehand, that's a problem. Moreover, it definitely sounds like the DRK wasn't doing their opener correctly; unless one of your DPS was a NIN and was using Shadewalker on him, the DRK should open with Unmend in Grit, use a Dark Arts-enhanced Power Slash combo alongside a DA-enhanced Dark Passenger, and then drop Grit and start DPSing. That kind of opener generates a massive aggro lead and would be more than enough if your PLD friend was vokeshirking him regularly, but if he didn't do the combo, then he wouldn't have started with a good enough lead. Frankly, it sounds like they were trying to mimic WAR's opener, which only begins with Tomahawk and Equilibrium before switching to DPS stance, but DRK can't spike aggro like WAR can.
That, and their inability to hold aggro, failure to use Provoke effectively, and general failure of the mechanics suggests they didn't know what they were doing.