In that situation, I would be very kind with her in the process of talking about the situation. I understand what her process may be, and that she may be genuinely concerned -- but her first port of call is to talk to the other woman, not involve an agency that could remove her children from her home. While her heart may be in the right place, her actions were very dangerous for that family.
I think that -- for me at least -- it's important that people use CPS judiciously for a variety of reasons.
First, AP families are largely suspect in many jurisdictions. Cosleeping can get children removed from homes. In some countries -- such as with a family in Spain -- breastfeeding past one year can create a removal. And an online friend in So Cal is currently separated from her children because she has a lawsuit against the city where she lives (for housing issues). Once you make that call, you no longer have any control -- and if you don't know what the local politics are, the children could be removed for a reason that you *support* (eg, you think it's great that she's co-sleeping!) and not the reason you called (you dont think it's great that she didn't get medical care).
Second, CPS resources are -- like many government agencies and other non-governmental groups that aid families in various situations of need -- finite and often stretched. To be honest, they have to follow up with every call -- whether it's because of this woman's fear, a frivolous "i'm getting back at you!" behavior, or a legitimate cause. This means that things get lost, overlooked, overblown, underblown, and ultimately, people do not get the resources that they need.
CPS has an important job, and an important purpose. It is important that when we report, we are reporting for a legitimate reason -- not our fears -- and that we are doing so in order to help a family. If that person is a friend, I woudl talk to her first in order to help her, not go to an agency. If that family is in need of help, and we are not friends, or if that child is in need of help, and i think the situation is risky, then I would absolutely go to CPS -- no conversation, no question.
At the end of the day, I believe it must be done with the family in mind and how CPS can *help*.