How do I tell my employer I think her child has cystic fibrosis?

Cystic fibrosis. It is very hard to diagnose cystic fibrosis without evidence of cystic fibrosis like sweat chloride test or the genetic testing. You can tell him/ her that based of what you are suspecting, you might need to ask the doctor about cystic fibrosis.
It depends... It really depends on your relationship with that person and on your ability/knowledge in recognizing cystic fibrosis just by looking at her child. It is not easy. Most likely, you could mention your concern and recommend a consult with a physician.