Let's start all over. Although reported as being a "lawsuit," what has actually happened is that one decedent's wife has filed a "government tort claim" with the County, as is a prerequisite to filing a lawsuit under the California Government Code. Such claims MUST be filed within six months of the injury or death (or up to one year if leave of court is granted on an appropriate motion after the six months has expired to file a late claim). The County will likely deny the claim, and then, maybe, a lawsuit will be filed.
In this particular case, the decedent was an employee of the County. Typically, an employee cannot sue the employer for on the job injuries or death unless the injury arises out of and in the course of employment (AOE/COE in worker's comp parlance). If workers' compensation applies, it is the EXCLUSIVE remedy against the employer.
That is not an open and shut question here: case law holds that Christmas parties, picnics, dinners, sports activities, etc. are not AOE/COE UNLESS attendance/participation was a condition of employment. However, in this case, there was also training going on concurrently and in the same facility, and this might be enough to bring the case within the scope of the comp law as establishing either conduct incident to employment and required attendance. "Damages" under the comp law for a death are set by statute.
If Comp does NOT apply, then this becomes a run of the mill lawsuit. The complaint will not set a specific demand for damages (that is prohibited by statute). Now I have not seen the claim, but as far as I can tell the lawyer has not stated precisely in what matter the County was "negligent." He will be required to do so in the complaint. A claim of deprivation of 2A rights is extremely unlikely, and even more unlikely to prevail, and for several reasons. First, the government owes no duty to any individual citizen (without a specific undertaking) to protect any individual from criminal misconduct. There is a specific governmental immunity in the Ca. Government Code so stating. Second, issuance of a CCW in California is discretionary, and it would have to be shown that the decedent had applied but been denied, and that the denial was an abuse of discretion by the Sheriff, because without one he could not carry anyway. Third, establishing gun free zones is a discretionary (policy making) endeavor of the County and/or State, for which there is a specific governmental immunity.
A more likely claim is a dangerous condition of public property. A premises liability liability claim requires that the premises were in a dangerous or defective condition, and that the governmental entity had notice and an opportunity to cure that defect prior to the injury. I think it is safe to assume that no such showing can or could be made here, given that the terrorists weren't on any watch list.
Obviously I am just speculating at this point, since no specific claim or complaint has been filed, but right now I don't see any substantial probability that a viable claim can or will be asserted. And there are other defenses that can be asserted, including the particularly important fact that the attack was an unforseen and unforeseeable criminal act by the attackers, attackers who were not acting in the course of their employment, and for whom the County will have no respondeat superior liability.
I wrote that it's not a major stretch to "argue" ... I didn't say you'd win the argument.