Now, the other posters on the board largely echo my feelings: hitting a baby is NEVER okay, don't excuse it, and take some responsibility as a mother for your choices.
If there is one thing that I will teach my daughters, it is to have high expectations for the men that they will marry. It's a message for everyone. When you are getting into a relationship and planning to spend your life together, you know to know, with rock-solid certainty, that no matter what happens, you and your future children will be safe. You need to know that your partner will always love and treasure your children, and would never, EVER do anything to mistreat them.
When I had our firstborn, my husband learned to care for her the same way that I did. We were in the hospital together, took basic instructions from the nurse, and then figured it out because that's what parents have to do. Books and courses won't always tell you what to do with a baby that won't settle between 1 and 5 a.m., or a baby that has a massive poop so explosive that it reaches her neck while you are traveling, or a toddler that vomits on your head in the middle of the night, etc. You deal with it, and you make sure that the other parent is someone capable of dealing with it too.
I'm happy to report that I have that sort of confidence in my husband. He may be tired or lack patience in other situations, but it was crystal clear that our children were/are his life. When our firstborn wouldn't settle, he would take her and gently start crooning "Can't Help Falling in Love" and other songs to her. Another time, he got her to settle by gently rubbing her tummy. I've seen my brothers-in-law act the same way. They always showed that they were patient, gentle men who were just meant to be daddies.
That brings me to my next point. This lady, and those like her, need to take the blinders off. You just can't say that a man who is "not violent or abusive" spanks a 5 mo old baby. Open your eyes, and stop making excuses. Lots of people get tired in the middle of the night. Lots of people hate the sound of non-stop crying. Lots of people don't always know how to stop the crying. None of that is an excuse. Decent parents deal with the frustration and know that there are some lines that you just don't cross. You shouldn't HAVE to explain to the father of your child that spanking a baby is wrong.
I was doing child protection work before I had children, and had a few "boyfriend beat the baby" cases that broke my heart. I'd hear moms say that he did it because he was frustrated because she couldn't have sex for 6 weeks, or because he couldn't stand the crying. I'd correct them, saying that he did it because he was obviously violent and immature, and needed to be locked up away from children.
UPDATE: At page 39, the situation starts to make more sense. Mom shares that she herself was physically and sexually abused until the age of 17 - which seems to explain both her insistence that the husband was "not abusive" and her idea that she wanted to be in total control over the baby even before the birth. I hope this is a wake-up call for some major counselling for BOTH of them. It also reaffirms my view that it is important to not just condemn abusive behavior, but to ensure that children grow up with functional, non-abusive role models.