http://seoul.usembassy.gov/esta_info.html
Looks like South Korea is in the VWP and she can use ESTA.
There is a chance she'll be denied that though since she has been denied a Visa before. If that happens she needs to get an visitor Visa from the embassy. (At least at the German embassy that is a matter of a week. In your case I would do that before booking a flight) I don't know how much of a fit they can throw about that. They might be afraid that you'll spontaneous decide to get married. You should be able to find how people have dealt with that before on here though.
Edit:
Since you've already applied for a K1 they might not even be worried about you getting married on a tourist visa/esta. If you get married without the k1 though she needs to leave the country again or she'll very likely get in trouble if you try to file AOS since she came on a non immigrant visa with the intention to stay. If she left after getting married you would need to file for a cr1 (which is what my wife and I did and that's a long long process. We applied in April 2013 and I'll go pickup my passport in half an hour now).
DSB got a reaction from chitraveler in Tourist visa after denied student visa