I hear you... I really hear you that this woman is dear to you and that you want to stay friends. I'm going to try to keep that in the front of my mind while responding.
But she really raises my hackles with her "assessing" another's child, especially the bit about her asking the child if she wants to go to school and make more friends.

The only advice I can come up with is for you to sit down with Mary and be very honest. Start off telling her how much she means to you, that you love her and treasure your friendship.
Tell her that you respect the work she does and the passion and devotion that she brings to classroom teaching. But tell her in no uncertain terms that there are more ways than one for a young person to grow in knowledge and that homeschooling is one of those approaches.
Tell her that while you acknowledge her opinion, you do not share it and that the only way for the two of you to remain friends (which you very much want) is for each of you to avoid the topic.
Tell her that you will not allow her to quiz your child in any way.
In return, you will not go into her classroom and quiz her students.
The irony, of course, is that if you did quiz her classroom students some of
them would be "behind". That is not a reflection on her so it shouldn't be a reflection on homeschool teachers.
Follow Mothering