Upvote:1
South Korea citizens generally do not require a visa to enter the US, and may enter under the Visa Waiver Program (this presumes she has no relevant criminal convictions/etc).
When entering by land an ESTA is NOT required. Instead she will be required to fill in an I-94W form at the border, be finger printed, and have her photo taken - which will entail a trip into the "secondary" area, although the entire process is really just a formality.
There is a US$6 charge for the I-94W.
To save a little time she can apply and pay for the I-94 in advance on the CBP Website, up to 7 days before crossing the border. She'll still need to go to "secondary" for the photo/fingerprints, but it'll be a little faster.
Upvote:3
She can get an ESTA wherever she can find an internet connection, including in Canada, but she does not need ESTA to cross the land border. I have heard that it makes the crossing smoother, but I've never crossed the land border with a VWP traveler, so I do not know exactly how it works.