The glow plugs turn OFF once the starter fires. IF was cold outside and it ran rough for a few seconds at idle after starting that's one thing but to run rough at idle continuous is NOT the glow plugs. Most shops remove the intake manifold to change the glow plugs on that engine. While it may be possible to do that & the filters in 2 hrs I'd think most shops would charge more. Might could be a mistunderstanding to whom he paid and who did the work as well, worth checking on.