If he changed the locks, the police would force him to let her back in and he'd be in court for an illegal lockout, which wouldn't help his overall case.
If he turned off the water, she would report the issue to the city and they'd force him to turn it back on, and he would likely face a fine.
His only option is to fight it out in a court that is heavily tilted in favor of tenants, and she (his "tenant") knows it.
I think at this point police may not do anything. Even he changes the locks. In SF and LA and few other metro area, they tend to ignore anything that can be a “civil matter”
If he turned off the water, she would report the issue to the city and they'd force him to turn it back on, and he would likely face a fine.
His only option is to fight it out in a court that is heavily tilted in favor of tenants, and she (his "tenant") knows it.