First, it's not just the last 6 years. Second, it's any country she lived in outside her home country, for more than 12 months, if she was 16 or older when in that country.
I was in the same situation last month. I am an Indonesian who live in Japan. I was able to get the Japanese police certificate, but not with Indonesian one, even Indonesian embassy in Japan couldn't help me. In the end I have decided to email the US embassy and they replied I need to check this site https://travel.state.gov/content/travel/en/us-visas/Visa-Reciprocity-and-Civil-Documents-by-Country.html and follow what is written there. Fortunately, the site written that it is not available, so the US embassy told me it is not necessary to obtain the Indonesian police certificate. But for South Korea seems it is available, so I think you or your partner can try to email US embassy and also ask her to try to contact the South Korea embassy where she live.
For example if you need to obtain a Japanese police certificate but you are not in Japan anymore, the Japanese embassy where you live can help you obtain it and they will cooperate with the Japanese police in Japan, but because of that process, it takes about 2 months to finally you hear things from the Japanese embassy. (experiences that I heard from my friend).
You can follow this link and click under Police, Court, Prison Records to find country-specific ways of getting what she'll need: https://travel.state.gov/content/travel/en/us-visas/Visa-Reciprocity-and-Civil-Documents-by-Country/SouthKorea.html
kevinfalting reacted to SteveInBostonI130 in Obtaining Police Record