The "law firm" involved has filed thousands of lawsuits on behalf of 3-4 plaintiffs. They travel all over California and target small "mom & pop" stores for minor violations, then demand over $10,000 to settle or face skyrocketing legal fees. There are numerous articles on-line about these unscrupulous attorneys, but is there any defense? The problems were minor and were readily fixed, and the plaintiff did not even complain at the time. The first notice of any problem was when the complaint was served
The Anti-SLAPP statute was enacted in CA to directly address and prevent these types of lawsuits. The anti-slapp motion can be filed along with a demurrer or motion to dismiss at the very beginning of the lawsuit. After dismissal of 5 or more lawsuits filed by the same plaintiff, they can be designated a "frivilous litigant" under the applicable statute.
I recommend that you retain an attorney but also contact your business's professional trade association. Unfortunately some individuals have made a practice of filing lawsuits of this type. I believe that recent state law changes made the type of action that is being filed against you more difficult to prosecute as a private party. You may want to repost this as a business law question.