Common warts tend to occur in hands and feet, and in other areas like elbows or knees.
They are caused by so called cutaneous HPV and these types of HPV do not have a cancerous potential.
Warts that appear on the face, forehead and arms and look like a reddish-brown or flesh-colored, slightly raised bumps, are known as flat warts. When the affected person has normal immune system, there is no risk for cancerous transformation.
Your mother even if there is no risk for cancer, should see dermatologist for treating the warts!