A.R.
You can breath through blankets, and get enough O2 to the brain doing that. My son used to, and still sometimes does, like to sleep with his blanket over his face. If I had covered my child for some reason, and someone freaked out on me I would be pretty annoyed - considering all the unwanted advice most parents get anyway. If you were going to say something, for fear or concern, I would do what one of the others had suggested and ask to see the baby or engage the mom that way - telling a parent they are essentially killing their kids is not a great way to get through!