satying after i-539 expired

[manessa]

my mother in law came to US on may1st 2016 and her i-94 expired on november1st 2016. we applied for i-539 and got approval till april 1st 2017. when does she need to exit USA? we have her flight booked on april 21st 2017. can she stay till then? Or can we apply to i-539 now again to extend for one more month?Or she need to leave before april 1st?
Thanks.

[newacct]

Either she leaves before April 1, or she applies for extension of status again with I-539 and have it received by USCIS before April 1.