Bad water at Mom's house

If this isn't the right sub, I apologize. My mom lives in Florida, and her in home water filter system is not working anymore. It may be the cause of her and my sister being ill. They have stopped drinking the water but still use it for showering, but they are both still dealing with diarrhea and no appetite.

The company that installed the system lied about what is contaminating the water. They claim high chlorine levels, but home testing proved otherwise.

I'm going to visit in a couple of weeks. What can I do to prevent illness aside from not drinking the water? Probiotics, maybe?