About 6 months, but remember don't let the US officer feel that she won't go back to Canada. They have the right to refuse anyone enter US if they feel he/she want to "move" there without any valid immigration visa. Please don't bring too much "living" stuff to US in case they check her luggage. Also, they will ask her how long she'll stay in US and I suggest to say no more than 2 weeks unless she has other good excuse need to stay longer. Mostly, they won't put the date limit on the passport.
Good luck.