You can do this. You have to call Delta and ask them to search for you on Korean. I believe she will have to connect through LAX to get on the KE flight to NRT. You might have trouble finding availability on DL from LAS to LAX. If so, just book it from LAX and buy the one way to get there. I think if there is medium availability to LAX and low on KE to NRT, they will try to charge you both awards. Good luck.