If she was Finnish, I'd buy it. We have tons of people of Finnish decent around here and have met more than one woman named
Saara. So it's not totally unusual to me, though I'd be a bit surprised to hear it on someone who wasn't, like, a Keskimaki or a Loukinen or an Erkkila. I think it's a bit... lame, maybe is the word I'm going for, to take on another ethnicity's spelling for what essentially is just kicks.
I'd recommend
Sari if she was bent on a change. It's still Finnish, but could passably be considered a nickname or pet name for
Sarah. Sairy or
Sadie could be options as well if she wanted a change from her usual.