Biometric appointment for B2 extension after US departure, should we send withdrawal letter?


New Member
I applied B2 extension of stay I539 application for my Mother on May 4th 2021 (her I94 was expiring on 3rd June) to be extended until 3rd Oct. My Mom is now leaving to go back to India on the 23rd of July and we have received a Biometric appointment notice for the 28th of July. Her travel plans cannot be changed due to unavoidable reasons.

Need advise please ,
1. Should I send a withdrawal letter via mail for the submitted I539 application once she leaves (after 23rd July) ? what documents should I attach ?
2. If I submit the withdrawal, will her stay between after June 3rd be considered out of status ?
3. Will the withdrawal create any issues during her reentry in the future ?