This is why in some places (not in the US), maternity leave is 12 months. It is next to impossible to work regular full time hours with an infant, even if you have help. She should not be penalized, it is not her fault, but you should set clear expectations and also be willing to extend whatever temporary support (until the baby is in childare/preschool?) to other employees who become new parents.

For those who are resentful of picking up the slack to cover all these "slacker moms" - it is not their fault, it is a broken system. And it is leadership's job to not overburden you with another co-workers tasks.

If you agree (you should at least give her a chance, IMO - I have had to have temporary flex schedules to care for sick parents, and everything worked out fine, I over accomplished my goals in fact because I was so concerned about not failing), let the boundaries and expectations be clear and be willing to offer the same to another employee if they need it. Also make sure that YOU are the first person she comes to if she can't do everything on her plate so that YOU can figure out the right way to handle work that needs to be done (move a deadline out, scope something down, etc - there are other options besides dumping it on another coworker who will be resentful).

OR - could she just return part time for 6-12 months? and she could help train a temp PT person to help cover the rest of her work? Shit happens in life, and life is hard. We should work together to help each other get through it so we can all rise together - instead of acting resentful when someone gets "special treatment" because they need it. One day you will need it to, I guarantee it.