The source of phlegm can be from conditions from the nose, lungs and stomach.
Common causes include:
1. Nose & sinuses - Common cold, Allergic Rhinitis (sensitive nose), Sinusitis (sinus infection) etc
2. Lungs - bronchitis (inflammation of the lungs), pneumonia (infection of the lungs), smoker's lungs etc
3. Stomach - acid reflux from the stomach
One of the most common reason for phlegm in the throat that I see in my practice is postnasal drip (mucous flowing down from the back of the nose down to the throat).
Quite often, patients with postnasal drip may not be aware of it and may deny any nose symptoms. Your mother can visit a General Practitioner first to help determine the likely source of her phlegm. If she does not have symptoms in the chest/lungs, she can visit an ENT Specialist for an assessment.
Hope this helps and all the best!