Your mom can leave anytime she wants.
Biometrics in India and inside USA are two separate things.
Once she leaves US, her pending B2 extension will be automatically abandoned by USCIS.
There is no need to appear for Biometric in India for this pending B2 extension as it will anyway be denied after her exit.
There are no bad consequences on future entry to US either. Don’t worry.
Her stay in US after i94 expiry till she leaves while application was pending will be counted as ‘period of authorized stay’. Its legal.