User blog comment:FairyRaven/Fairy Tail Chapter 348: Return of the Demon, Review and Commentary/@comment-7418318-20130823094049/@comment-7418318-20130823134947

And that Lucy showed concern and sympathy for Flare even after getting abused by her probably helped to speed up Flare's recovery. And the offer of joining FT brought her the rest of the way.

While it is an unusually fast recovery, I wouldn't call it impossible even in reality.