Upvote:3
There is no rule on how long they have to be back in Canada before they can go back to the US on a visit. So, they can go back for 1 day and then try to go back to the US. On the other hand, there is no guarantee that they will be admitted for 6 months the second time, or that they will be admitted at all. Admission of foreigners is at the discretion of the immigration officer, and for B2 visitors, how long to admit them is also at the discretion of the officer.
So, yes, it is possible that they go back for one day and are admitted for 6 months again. But it is also possible that the officer decides that they've been in the US too often and only admits them for 3 months, or 1 month, or 1 week, or even denies them entry altogether.