No always. The CBP agent always asks several questions, such as:
Are you a resident in Canada? Why are you in Canada?
How long have you been out?
The same situation is for either by air or by land.
Most of the time, enter by land may be easier.
Generally, if she stays out of US for over one year, she needs re-entry permit (I-327) to enter US. GC is not useful under this situation.