I think that in order to "reset" her stay your Grandma must hand in her I-94 slip when exiting the US at the border in TJ so when she returns to the U.S. she would need to get a new permit and a new stamp with another 6 months. If she doesn't hand in the slip, immigration at the border will see it and just let her in again without stamping her passport and the original entry date still running.