Not like me to be posting in the Citroen section - haven't done so since I had the C5

Anyway, my grandmother just called me asking my advice.
She just arrived home from Donegal (a 120 mile journey she does once a month to see her mother) and said that towards the end of her journey the car warned that the Depollution System is faulty.
The car is a 2005 Citroen C4 Coupé 1.6 HDi VTR+ with 50,000 miles.
From my limited knowledge I know that this is a fairly common message for HDi drivers to get, and relatively simple to put right?
Will it require urgent attention, or can it wait until after the festive period?
Thanks!