It means a new mirror unfortunately if they have tested the wiring, as there are no replaceable parts for the mirror apart from the glass and the shell cap.
You can get these from Citroen of course, but you may find you can get an aftermarket mirror - but you need to be exactly sure which exact mirror you have as these differ depending on trim level and options. If you get the wrong one it won't work as the electrical connections won't be correct. Apart from the electrically adjusting, electrically folding, heated mirror options, some have puddle lights and some have blind spot monitoring diodes in the glass if you have that option....
https://www.wingmirrorman.co.uk/wing-mi ... rror-units
Or look for a second hand one. Aftermarket ones will likely need painting, unless you can swap the shell over (assuming it fits).