For your friend it is a time of huge change - I was working from home before I fell pregnant, so the transition wasn't dramatic - but for your friend, moving from the workforce, earning money, being stimulated by outside activities, feeling as tho she contributing, socialising (even if just around the water cooler) ...
... and then going to being stuck at home with a screaming baby that doesn't sleep when you need to ... no social life ... no interests ... isolated during the day when everyone else is at work ... no matter how wanted the baby was, how much it was loved, very easy to start regreting what you've given up ... realising this child is dependent on you for the next 18 years ... longing for your old life back ...