I'm sick just reading your post. I would end the friendship. I wouldn't even bother explaining myself. Unless you felt like there was a way to be a "witness" to her of GBD, but I would guess that she's pretty much set in her ways and probably not open to suggestion, iykwim. I'm sure the Lord has another way to provide you with the nutritional help you need without having to expose you or your children to that.
I'm so sorry!