Crossover is actually an electronic filter. In audio systems, a crossover is used to separate signals to drivers (speaker). For eg. 20Hz-40Hz to woofer, 40Hz-4Khz to Midbass and 4Khz-25Khz to tweeter.

For your gf's case ... if I'm not mistaken, the output is connected to 2 speakers. Normally in lots of cases , midbass and tweeter together. For this, the midbass will driver whichever frequency its capable of and same goes to the tweeters.

Hope this helps