Granted, it was a crackish RP, so she just woke up in an alley, and was startled when she realized she was alive. Thought it was just a very real dream until she did the stereotypical arm pinch, and even looked to see if she had her scars (which she did). If I had to explain it in non-crack terms, I would say that it would be almost like granting a wish or somesuch; that at the end of AC, when she and Zack walked into the light, suppose they were given a wish before returning to the Lifestream for good, and Aerith's was to be able to be with her friends for just a little while longer, while they're happy and not fighting against some world-ending catastrophe.

Though for this, I would definitely look into the Bleach gigais (moreso since I don't follow Bleach l-lol) or theoretically come up with a way it could work in the FF7verse without the use of Phoenix Downs, since it's been proven those don't work on her, *lol*