Well, that's the thing. At that time, she's just getting into Alchemy. Perhaps the trick to get her to join the Allied Forces would be to impress her with a show of arcane might (aka bust out some BLM moves for her). That could get her to want to apply her magical arts one last time before she becomes the Alchemical expert she is today.
Or, failing that, try taking a high level Black Mage that has a high level in Alchemy to go talk to her? You never know.
