invite etiquette...

OK...so we have a family friend we are inviting and here's my question...

She got married later in life (late 30s) and had decided to change her name from 'Smith' to 'Jones' when she had kids, but the first kid came within the first year of their marriage and she REALLY doesn't like the name 'Jones' so she has never changed, refuses....

SO...on their invite I know to list them alphabetically, I know that to be proper, I should use the name she uses (she doesn't even use 'Jones' for casual purposes...), do I call her Mrs. Smith or Ms. Smith? I'm thinking it's probably Ms. Smith...but I am hoping for some input. Thanks!!!

My SIL is a professor and she uses her maiden name, never took her husband's even after having children.

This is how I addressed their envelope:

Mr. Steve Jones and Ms. Debra Smith
123 Main Street
Anytown, NY 11111

She can't be Mrs. Smith because Smith is her maiden last name other wise she would be her mother and Mrs. denotes that you have taken your hsuband's last name.