One word - teenager.
Doesn’t change anything (pardon the pun). At the point SAD handed off the baby to her friend, she should have had a good three weeks of experience handling the baby already. Babies usually need to be changed six to 10 times a day. Easily, SAD should have already changed that baby more than 100 times. Unless stinky little Barry was constipated for a couple of weeks, she should have already had changed a poopy diaper at least 15 times or so.
Also, we’ve been told that SAD wasn’t the typical teenager. She was more worldly and confident. Handing off a baby to change a stinky diaper doesn’t fit the profile.