[She gives Zack a brief smile for pulling out the chair before she sits down...

And Lightning's words have her grip the sides of it hard as she looks down, pushing back grief.

...She hasn't asked (or told) Zack to leave, though. Which means... he will be told. That's how Aerith takes it. After a pause, her eyes rise to Lightning's and she says:]

Light is a l'Cie, Zack. [she turns her head to look at him.] l'Cie are people chosen to complete a certain task. They're made stronger, given their own magic... but they only have a limited time to get the job done. ... I was trying to buy her more, but it just made things worse.