When you buy a guitar, (hopefully) the neck is set to guage of the strings it was originally strung with. If you want lighter or heavier strings your best bet is to take it to your local shop and have your neck set for what you wish.

Most accoustics are originally strung with poor quality 13's which can stretch over time on the wall of the store.

I'd find a string that she's comfortable with (over the first couple weeks / months) and then have it set up for those. Your intonation will be better and they'll stay in tune much longer.