You may take the date till which she was paid salary as the last working day. For example, if she was given maternity pay for 84 days and then she remained on privileged leave for another 30 or 36 days depending upon the availability of PL to her credit and since then she is absconding or writes expressing her decision to resign. In such a scenario, take the date on which her PL expires as the date of exit for all purposes including issuing service certificate. Naturally, for F&F only gratuity (if applicable) will be available to her and that can be calculated based on her salary as on the above mentioned date.
Madhu.T.K