Hmmm... As you implied, she may not need correction at all, particularly if she's not diving seriously yet (and I note your other thread about what kind of lessons might be available to her). Refraction works a lot differently underwater than in air, and is much more forgiving; but I'm not sure it's a function of magnification.
That having been said, it's tough to know until she tries correction.
Particularly with her correction, I certainly wouldn't go with expensive custom-made lenses. I assume not much if any astigmatism?
Does she have a mask now? Are there off-the-shelf lenses available? An Internet search would determine that easily. Off-the-shelf lenses would work just fine, and are available for like $35-$40 each, depending on the mask. They're available in half-diopter increments, and you'd want the closest correction downward... or -1.5 in her case.
Me? I have about -10 diopters of correction in each eye, with serious astigmatism and reading correction. I see perfectly underwater with -9.5 lenses in my mask, with just spherical correction (no cylinder or reading correction).