Where in Boston are you located and what is the name of said shop?
Not to be NEGATIVE, however, did your friend NOTICE if the car has NEW FILTERS in it now?
An UNETHECICAL type shop could do such a thing and then do the glow plugs for the added PROFIT.
I am a born skeptic (however that is spelled)
Rough idle could be anything from AIR in the fuel lines to poor fuel injection to erratic compression (not likely) as the car runs well now to dirty filters and poor fuel.
I am on the north shore and that is the reason for the question.
Thanx, and keep those diesels humming!~