You see, that's the problem I have with many of the posts on this thread. It's all about the woman, and her marriage, and problems that might ensue if she goes behind her husband's back.
What about the child? If that letter is to be believed, that baby is genuinely in danger. As someone else posted earlier, get the baby into a safe environment, then sort the rest of the stuff out afterwards.
Taking the baby to a hospital and dropping her off would be a whole heck of a lot more constructive than sending some stupid anonymous letter to Dear Abby and getting Abby's pablum back in return.