Dolphin Tale 2 is not a completely true story. It takes some inspiration from real dolphin conservation efforts and stories, but the plot and characters are largely fictionalized to make for an engaging movie experience.
Yes, it is. The real - life events that inspired the movie are quite remarkable. Winter, the dolphin in the story, had her tail severely damaged. People worked hard to create a prosthetic tail for her, and her journey of recovery and adaptation became the heart of this wonderful story that was then made into the movie 'Dolphin Tale'.
The mermaid in the fanfic might be an outcast in her own mermaid community. She discovers the story of the dolphin in 'Dolphin Tale' and decides to use this as an opportunity to prove herself. So she secretly helps the humans and the dolphin, while also learning about the human world and the value of friendship along the way.