I'm curious - since you state she didn't know Sephiroth was there, why is Aerith going off on her own dumb? Obviously she made it to the Forgotten City on her own just fine, and if Sephiroth indeed hadn't been there then she presumably could have called Holy and been fine. I think she went off on her own because A - it was a long journey, considering they were near the Temple/Gongaga when she left, and B - the rest of the group was likely worried about Cloud.
BTW, random whoring - my first FF fanfic ever was Aerith having an encounter with Sephiroth on her way to the FC.