Outrageous! - their bungling investigations caused this - so they should pay.
Surely just disconnecting the horn should not cause the whole pcm to shut down permanently?
You need to be asking them:
Assuming the old alarm/horn needs disconnecting before fitting a new one, do they really fit a new pcm every time as well?
Is a new pcm required every time an alarm is replaced?
This is just BS - at your (considerable) expense - IMHO!