I brought her back

Sunset Shimmer went to the Pony world while holding Flurry Heart in her arms. Twilight, Cadence, and Shining Armor were thrilled and happy to see Flurry Heart brought back alive. Sunset explained that she found her at the school and took her in to show Twilight's Canterlot High Friends who also wanted to help. She also explained that she was looking after her ever since and knew when it was time to take her back to Equestria. 

The only pony who didn't seemed to be amused to Sunset's appearance was Starlight Glimmer, because she didn't trust her. Starlight studied on what the human world was like and thought that all of those two legged creatures were killers and steal the land for themselves. Since she's secretly thinking it through.

This was going to be very difficult for Sunset to say good bye to Flurry Heart, since she became very close to her since she's been looking after her. She also explained to Shining Armor and Cadence that they are welcome to visit her world also with Flurry Heart by going through the portal in the mirror.