I think if their child is going to the school-- then they have a stake in the school-- if they no longer go to the school-- then no and our by-laws state that they have to be a parent or guardian of a student at our school in order to run for a position.
It depends entirely on your bylaws. Most groups tend to feel that officers (if not all members) need to be direct stakeholders the school involved. This usually translates to being parents/guardians of currently enrolled students or staff employed at that school and having the bylaws state that requirement.
So your bylaws define what a MEMBER is and then further what might be required to be an OFFICER.
Some organization's bylaws are wide open - ANYONE can be a member, or perhaps even an officer. Or they might leave membership open to all but further define requirements to be an officer.
Your bylaws are the rules by which your group operates. So that's what should answer this question.
If her children still attend the school and it isn't in contravention with your bylaws then I would say yes. We have several families that because of our school's reputation and our great PTO bring their children from out of district to our school. They have the right within out bylaws to hold positions within the organizaiton.