The electronics on these cars are very sophisticated. A circuit board component can easily be damaged by a simple flash to ground (neg) of another circuit during an installation. Having said that, it cannot also be proven....that that was the cause....because it could very well be just coincidental. I don't see where they can hold you liable for anything but the repair/installation that you performed.
Good luck.

edit:
Do you not have a disclaimer on your invoice or contract that states your liability is limited to the value of the purchase, installation or service you are providing?