With The Witcher Enhanced Edition (v1.4), It is possible to give Celina the Wreath of Immortelles, and her not attack.
At least, when done at night. I've done this twice, and both times she didn't attack. Both times, however, I don't know if she disappears permanently from the game, as I leave the Fields never to return afterwards, completing all quests within the next day.
I post this here instead of emails because my Yahoo account doesn't like the addy to report it. It bounces every time.