The union is required to provide support for the individual worker. To go after her, through the union, would likely require a specific complaint from a fellow officer and some unions have specific rules barring complaints against fellow union members. Without seeing the union regs and collective agreement, we can't know if that's the case, but if so then it would mean going through HR, and the officer in question would still receive union support.
Not the way I'd want things to work, if someone without proper training and qualifications was carrying a firearm around me, but likely how it goes.