You can refer them to this link that I believe is a good explanation of why pathogens do not generally develop resistance to chlorine.

The primary pathogens that are highly resistant to chlorine are the protozoan oocysts, Giardia and especially Cryptosporidium. This resistance was not developed through genetic mutation, but was always there as the cysts (with its contents being inactive) existed to be hearty in the environment in general. When such cysts enter a host, then their contents are released (probably when reaching the acidic environment in the stomach), multiply in the intestinal tract, and are then released through diarrhea.

The next most resistant are bacteria that form biofilms. Again, this is not resistance that was developed via mutation, but rather general resistance against harsh environments. Biofilms will mostly form when the bacteria adhere to surfaces so the key is to never let the chlorine level drop so that bacteria can be killed quickly while still free-floating before they develop thick slime layers in a biofilm.

Richard