I agree that before taking drastic measures, someone should go inform the children's librarian of the law. The momma this happened to could return, law in hand, and seek out the librarian. She then could say something like this.
"Hi. Remember me? I was here the other day and you had me go into the video room to nurse my baby? Well, I felt a little uncomfortable leaving my older son unsupervised. When I discussed this with a friend, she shared with me the Maryland law that allows nursing mothers to breastfeed anywhere they are otherwise allowed to be. Now, I'm sure you didn't know about this when you approached me before, so I thought I'd give you a copy of the law. Thanks!"
Then she could go with her son, and when the baby wants to nurse, go ahead and nurse. THEN if the woman says something, you pull out the big guns.
Remember how up in arms we were when a certain Lactivist received a C&D letter from an attorney about a certain trademarked phrase? And how a simple letter or email or phone call from the board might have accomplished the same thing without the negativity? Well, I think it's appropriate to give the same courtesy in situations like this.
If you want to go to the board to make sure they are aware of the law, that's fine, but I'm not sure having the first step be a letter from a lawyer is appropriate, given the description of what happened. I would think otherwise if she stated her rights under the law and was rebuffed, or if a supervisor backed up the librarian.