The usual left-handed adaptation (per, say, Jimi Hendrix and Paul McCartney) is to restring the guitar (low E where the high E is normally placed, etc.), and play in a mirror-image of "normal" right-handed playing. This seems sensible and "natural" to me.

The "upside down (and backwards)" approach, most famously used by Elizabeth Cotten, is to hold the intrument "left-handed" ~ right hand on fingerboard, left hand ready to strum or pick ~ and take it from there. All chord shapes will be reversed from the usual, which in most cases makes the chords harder to make, or so it seems to me. Also,if you want to fingerpick (as did Libba), you have to keep up the regaulr beat of the alternating bass wsith your index Finger while playing the melody and/or treble part with your thumb, which also seems counterintuitive to me. (Of course, I'm prejudiced, after years of playing right-handed in the usual manner.)

It should be noted that. for a natural righty playing in the conventional "right-handed" manner, the left hand is required to exercise at least as much dexterity as the right, if not more. For a person starting completely from scratch, it should be just about as easy to learn lefty as righty. Assuming that your friend has full use of all fingers on the right hand but limited use of the left, I would tend to set up a guitar left-handed and have her fret the strings with her good right hand and to strum (and perhaps eventually pick) with the less-abled left hand.