There is technically not much you can do, as she will support the certificates from the doctors, that she needs rest and should not travel for long distances due to the surgery done while having the baby, So she will have a medical reason for you to releive her immediately.
Also as you would have appointed an tempepory person to work on her behalf, so handover question also does not arise. She has still one month of leave with pay for maternity and thereafter one month leave without pay, which she can anyways avail as per the MB Act.
So just accept the resignation and releive her as an expection case.
Regards,
Ashutosh Thakre