I don't think it matters why she chose not to go to the wedding. She RSVP'd no and she didn't go. She's in the clear etiquette-wise.
Now, when dealing with close friends I can definitely see hurt feelings if a friend chose not to attend a wedding for a trivial reason. But, I think having a baby is a valid reason to miss a wedding. First, she RSVP'd no before she gave birth. At that time, she had no idea how the birth was going to go, when it would occur, if her and/or the baby would be healthy, etc. Also, if she's breastfeeding, she'd have to leave often to do that...not to mention not wanting to expose the baby to germs, have babysitters so close to the birthdate, and the lack of sleep. I think she was smart to anticipate that she wouldn't be up to a wedding so close to having a baby.
As for her stating that she had 'cabin fever' and wanted to get out. I understand that feeling. After giving birth, I craved going out with my girlfriends and just returning to my old life for even a second. But, even when the opportunity presented itself, weeks after giving birth it wasn't the ideal choice for me to make. So even though I desired to get out, as a new, breastfeeding mother with a weeks old baby, it just wasn't plausible. So, I didn't do it...one of the many sacrifices I've made since taking on the responsibility of parenting.