You have to get help for this child. As mentioned, you are most likely legally bound to contact the authorities. I would not only file a report w/ Children's Protective Services, which can be done on line, but further, call the police as they will most likely respond sooner.
It does not matter why the parent is abusive. It does not matter that it does not happen all the time. A child is in danger. This child may have already suffered damange that can never be repaired physically. Do you really think there is any way this child has not suffered mental damage already?
This baby, and yes, 15 months is a baby, can not seek help. He or she can't even speak up for themselves. This breaks my heart. Please, please, please save this kid.
A parent who is abusive usually gets worse over time. Next time the baby might not make it. Even if he/she does do you realize each instance increases the chances that they themselves will become an abuser some day. Please put an end to this!