Wouldn't this be a better question for the UA or US boards?

A tip for using miles ex-EU is that DL doesn't collect the YQ (international surcharge) on KE flights (at least not yet). Thus, if your GF wants to fly FRA-ICN (and then anywhere beyond that KE flies), you can avoid the YQ. Might make for a fun trip to meet up somewhere in Asia, even if you're flying TPAC and she's flying over Asia. The alternative would be paid flights spaced as far apart as possible and then the awards stuck in the middle, so that the origin shifts to the US. As long as the first award return doesn't cause you to violate the minimum stay rules on the paid ticket, I would think DL won't care.