The Craft

Sarah Bailey (Robin Tunney), a troubled teenager, has just moved from San Francisco to Los Angeles with her father and stepmother. At her new school, she forms a friendship with a group of girls rumored to be witches, Bonnie (Neve Campbell), Nancy (Fairuza Balk) and Rochelle (Rachel True). At the same time, Sarah becomes attracted to the popular Chris (Skeet Ulrich). Bonnie, Nancy and Rochelle worship a powerful deity named "Manon".
Sarah exhibits supernatural powers from the onset of the film (revealed to being a "natural witch," i.e. born of a parent who practiced witchcraft) and her new friends believe that she will complete their coven, making them all-powerful. When Sarah is harassed by a vagrant with a snake (whom she had encountered before in her new house), he is immediately hit by a car and the girls believe that together they willed it to happen.
After a date with Chris, Sarah is upset to learn that he has spread a false rumor that they had sex. When Sarah confronts him, he treats her disrespectfully in front of his friends. Sarah casts a love spell upon him; Rochelle casts a revenge spell on a hateful racist bully, Laura Lizzie (Christine Taylor); Bonnie casts a spell for beauty; and Nancy for liberation from working-class poverty. It becomes clear that the spells have been successful: Chris becomes infatuated with Sarah, scars that Bonnie has on her back are completely healed by an experimental gene therapy, Laura loses her hair, and Nancy's abusive stepfather has a heart attack and dies, leaving a large insurance policy which makes her mother rich.
Nancy lusts for more power and encourages the others to join her in a rite called "Invocation of the Spirit". To do so, she plans to "call the corners" by creating a fire on a beach, so that all four natural elements (earth, fire, air, and water) can be called upon by the four separate girls, to grant them further supernatural abilities. On completion of the spell, Nancy is struck by lightning, and all four girls fall unconscious. Afterward Nancy is seen walking on water, claims to sense Manon in all things, lacks empathy and begins taking risks with her life and those of others.
The girls' spells soon bring negative consequences: Bonnie becomes aggressively narcissistic; Laura Lizzie is traumatized by her baldness and becomes hysterical; and Chris attempts to rape Sarah when she rejects his continual advances. Nancy uses a glamour spell to make herself look like Sarah, and then seduces Chris. They are interrupted by the real Sarah, who insists that Nancy leave with her. Chris is upset about being fooled, and taunts Nancy that she is jealous. Nancy uses her power to push him out of a second-story window, killing him.
Sarah performs a binding spell to prevent Nancy from doing more harm. The binding fails, and the coven turns on Sarah. They invade her dreams, threaten her, and use their powers of illusion to make Sarah believe that her father has been killed in an accident. They try to persuade her to commit suicide, before Nancy cuts Sarah's wrists herself. Sarah successfully invokes the spirit of Manon and is able to heal herself and fight back, demonstrating far superior power to her former sisters in coven. Sarah scares off Bonnie and Rochelle and defeats Nancy, binding her power to prevent her from doing harm. Nancy is committed to a psychiatric hospital and Sarah is the only coven member who does not lose her powers.

Question: How are the scars on Bonnie's back healed ?
Answer: They are healed by an experimental gene therapy.