fire them... you don't need the liability. If something happens, it will be your butt in court in a lawsuit!!
Send them a letter regarding non-compliance, return receipt requested so you get a signature, certified mail to the owner/operator if you want to completely cover yourself. List all the non-compliant issues you know of, then discuss the letter with the owner so they know you need to cover yourself because VGB compliance is not just a good idea; it is the law.
Bring the letter with you, ask once again if they would like to become compliant, if they refuse, note the date and time on your copy of the letter, get them to initial if you can (again emphasizing how serious of an issue this is) and tell them goodbye.
You will not be able to make enough money from this client to cover you court costs, much less the judgement awarded to a victim, should any (and I emphasize any) kind of accident occur at the facility. Federal safety law non-compliance is negligible, and the suit will be awarded to the victim, no matter if it is a stubbed toe or a drowning. By continueing to service the non-compliant pool, you essentially are agreeing with the owner's decision.
I agree with Wes, I dont even send them the letter. If I email them the estimate and they say no, I pull off the job and alert the local department. I shut the pool down as well. Its not the customers (of the commerical property) fault that the owner doesnt have the money to fix the pool up to code why should they suffer if they get hurt?