The visa is for entry only. Once the visa expires she can no longer enter the US but will need to apply for a new visitor visa in her home country
On entry using her visitor visa , she is granted a maximum stay period by the CBP officer. She must leave the US before that date ( find it on the I94 entry record associated with her most recent entry ). Even 1 day overstay will cause issues with a new visa application
If her visa expiry date is before the I94 “end of stay” Date, she can stay until the I94 date. The visa must be unexpired when she enters. It doesn't have to still be unexpired when she is leaving