My son has a daughter with a lady and they never got married. What is the proper way for me to refer to that lady? Obviously she would not be my daughter-in-law but is there a proper way for me to refer to her? Thanks.

Girlfriend, significant other, partner

Mother of your grand-daughter is honest, fair, and not misleading nor disrespectful of her role, whether she and your son are together or not. "my grand-daughter's mom....." fine. A first name is also acceptable. What sort of circumstances are you typically under when you need to address her mother?