Every choice she made that night was really stupid. Being drunk in charge of a newborn just the start. For most parents, who are still obsessed with making sure their baby's head is supported properly, throwing around a tiny 4wk old baby would be unthinkable.
Without common sense, any other decision would have been better for him that night. As hideous as it is, even leaving him in the caravan alone would have been a safer choice for the baby, and at least with that option the possibility of someone reporting a crying baby left home alone could have saved his life.