if I understand correct, and she will be fretting with the impaired left hand.,
Then I'd seriously recomend aquiring a good quality 3/4 scale guitar
[usually marketed as an older child/young teens instrument]
remove the outer 2 strings, and tune the remaining middle 4 strings
to mandolin interval G-D-A-E
An experience guitar shop worker should be able to provide the correct gauge strings to achieve this tuning setup.
The reason I suggest this is that she would effectively be learning mandolin chords which for a large part require only 2 fingers
to play most common song patterns.
A good 3/4 guitar setup with lower action neck setup and lower gauge strings
would also require less pressure to hold down chords
than most standard guitars and mandolins.

just an idea , anyway....