Can someone explain this about guitar and piano duets for me?

The most important thing is that, however you do it, you're both playing in the same key.

Song books and sheet music usually have right and left hand piano parts, the “top line” (melody) and guitar chords. Often a piece of piano music will include guitar chords too.

You do not need a song written for two instruments.  Any song can be played on guitar and piano.  If you are both beginners, she could strum chords on the guitar while you play the melody on the piano, then switch roles and let her pick out the melody while you play chords.  It would help both of you to find qualified teachers.

I'm no expert as I've not been playing long, but I'd expect that it'd sound better if you had a song written for two instruments so that you're both playing a different part of the same song, different notes that go together.  I suppose that one thing you could do with a simple, single note melody is to play seven semitones apart, as everything you played between you would make a power chord.  As in she plays an E and you play a B, she plays an A and you play an E, etc.  Obviously that won't work with more complex music but it's just a suggestion.  If you were doing that then the more bass instrument would play the lower note, the higher instrument would play the higher note.  One of you could add a note that's either three or four semitones higher than the low note to make a major or minor chord between you.  Of course you could also switch which one of you is playing the middle note for a bit of variety, or use full chords on one or both of the instruments too.