The regulations for a B-1/B-2 visitor, at 8 CFR 214.2(b), allow admissions of up to one year to be granted in that status. Six months is more of a minimum for a B-2 entry, if your mom asked for a long stay and the officer thought your mom seemed reliable and the purpose of the visit reasonable then the officer has the discretion to grant a stay longer than six months. If your mom requested to stay for a full 6 months the longer stay might have been granted to allow her some flexibility to avoid an inadvertent overstay or an extension request should her plans need to change.
As the comment points out it might be good to give your mom's details to the CBP I-94 website to make sure that the leave-by date on the CBP's record of her current I-94 matches what is in her passport. If it does she can legally stay until that date. The only other thing to consider before having her do that, however, is that staying significantly longer than she requested when she entered, if the officer noted that in her record (which seems likely for an exceptionally long stay), can cause them to have a more negative view of your mom's reliability on subsequent entries, so if you expect your mom to visit again soon you can minimize the potential for trouble the next time she comes by having her stick to more-or-less the trip she told them she was taking this time.