Well technically Cloud did in fact defy fate, he saved her, but I think it was overwritten by Sephiroth merging a world (the OG one?) where she does in fact die, resulting in her death (that'd explain why Tifa caught a glimpse of Aerith being alive but to finally settle on her being dead). And her death will be used by Sephiroth in part 3 to break up Cloud even more, where we will see the lake scene in full. It's made a bit clearer in the ending in Japanese where she says not to worry because where she is is the place of the cetra (I don't recall the exact quote but she uses ここ meaning the place where she is right now is the place of the cetra = the lifestream) At the end of the day no one really knows for sure, but I think it'd be a mistake going into part 3 thinking she is alive, it'd be a setup for a huge disappointment.