What causes phlegm and how can my mother get rid of it?

A little more detail is needed. What kind of phlegm, how often, and what are your mother's health conditions?

You may want to have start skipping dairy products and see if that will help. My father was getting phlegm and the doctor told him to get off dairy products. He has very little problems now with phlegm. There are alternatives to dairy products such as Soymilk, non dairy ice cream, non dairy butter, etc. Hopefully this suggestion will help you.