You already know she's allowed to stay for six months. You asked if she can leave and come back. Answer to that is yes. She can enter multiple times. There's no limit. However while she can stay six month on one visit, she cannot stay six months, leave for a day, week, or month and come back for another six months. She must be outside the USA more than in, even if only a day more. This is measured during any 12 month period.
She has a visitor privilege. Many Canadian Snowbirds actually live in their own homes in the USA six months out of the year, but they go leave for the other six months.