We know that Aeris and Zack were boyfriend and girlfriend, but it was never specified in the game that they broke up. So, that would mean that they were still together until Zack died which happened right before the game started. Also, Aeris was never told that Zack had been killed. You also see Zack and Aeris together in the movie. Aeris knew Cloud for only a short time before she died.
Well, what do you think?
Well, what do you think?