How can I have a lien disputed and removed?

I'm run a solar sales organization. I have a client who paid cash for Solar, Roof and a new HVAC unit with ducting work. The HVAC construction company I subcontracted fir the job was a total mess. He came out to give an estimate and while we were still negotiating the projects cost. He started the job without a signed agreement between us and without any form of pdown payment exchanged for him to begin the job. He installed the unit that he knew a part in the furnace was on a recall. So it didn't work. He left a whole in the drywall near ducting system. He left gaslines unvalved and then was unreachable when we were trying to get him to schedule to come back to fix all the issues he had left on the project unfinished. We had to hiring another subcontractor to correct the issues inorder to pass the HERS test. Which added to the overall project expenses. When we sent him an invoice of the additional charges to fix his mistakes and for having abandoned the project. We deducted the additional expenses from the verbal agreement he made with us at $12,500.00, we did not hear back from him. In fact we have still not heard back from him since. However, he filled a Mechanical Lien against the homeowners and their property for $14,000.00. The lien he filed is fraudulent we never signed a subcontracting contract with him or his company at any time during the process of the project and installation. The property is in Napa County but their court house says the lien wasn't filed in their courts jurisdiction. I need help getting to a resolution for both the homeowners and my company. Considering that this contractor has numerous suspensions and violations on his licenses. I have a feeling that the lien my have been improperly filed and granted. Is there a way to determine if he also committed fraud when having the lien put against the homeowners property? How can my company dispute the lien validity in order to have it removed and/or expunged?