There is no guarantee she will be given another six months stay. What she is given is entirely up to the immigration officer she encounters when she enters Canada. Any of the following could happen: granted the default six month stay, granted a stay shorter than six months, denied entry.
If she is spending a lot of time in Canada, immigration may think she is trying to live in Canada rather than just visit. In case she is questioned, she should bring proof that shows she has strong ties to the US (assuming the US is her home country) and is really only in Canada as a visitor. Strong ties to the US are things such as: property ownership, lease agreement, proof of employment, proof of school enrollment.