actually, answer me this. Aeris could have been getting over a past boyfriend... but what I'm trying to say is, it seems oddly coincidental for that boyfriend to be the same guy Cloud swapped identities with. I mean, there was a reason they wrote it that way.
It's also oddly coincidental that Cloud just happened to crashland into the hangout of the last living Cetra/deus-ex-machina-in-training. At the time of the game, there really was no good explanation for that except it was convenient for the writers, as it can be convenient for the writers to have a minor background character serve two purposes.
Aerith knowing Zack--instead of SOLDIER #57--is also very convenient because it can be hard to write characters being hung up over someone without making the audience roll their eyes and say "move on already!" Like if Aerith had actually been on the dot with her talk about Zack taking off with a girl, or if he'd died years and years back when he actually disappeared, probably a lot more people would think she's weak for being hung up over someone who was a jerk/looong gone. But Cloud & Zack's backstory give a convenient answer for why she'd just be getting over a boyfriend she hasn't seen in years: he was flirtatious but he wasn't a jerk at all, and there was hope she'd see him again until very recently.
Basically, getting more mileage out of a plot thread that's already been set up. And it helps add some emphasis to Cloud's identity crisis that someone actually knows where he's getting his quirks from. (Arguably Tifa would have served the purpose too since she did meet Zack...but it seems a little unrealistic for her to remember the mannerisms of a SOLDIER she wasn't all that interested in and just took on a tour once years after the fact.)