I know in the state of MA, if you clean a system that is out of compliance with NFPA 96 standards and don't notify the state fire marshal in writing, you are held liable.
He then contacts the restaurant and shuts them down till they fix whatever needs fixing.
A proactive State FM makes all the difference in the world with compliance issues.
When you've got a FM who will not only back you up, but shut down the restaurant until they are compliant, then you've got some teeth.
But to blame the hood cleaner when there are so many other obvious flaws in the system, is just piling on.
Now the point I'm making ASSUMES that the hood cleaner actually cleaned the grease exhaust system. If you didn't and charged for it, then you don't have a leg to stand on.