Well, in a story, the friend might be playing a prank with some new technology they don't fully understand. There are some fictional technologies that can manipulate the body's physical form. Also, the friend could be under the influence of a curse or a hex that causes them to involuntarily turn others into different genders. It could also be that the friend is exploring some sort of spiritual or esoteric practice and makes a mistake during a ritual that results in the transformation. For instance, if they are trying to communicate with a gender - changing spirit and things go awry.
One possible reason could be as a form of punishment. For example, if a character has been misbehaving in a way that is considered inappropriate for their gender, magic might transform them to teach them a lesson. Another reason could be an accident. Maybe a young wizard was practicing a spell and it went wrong, resulting in the transformation. Or it could be a magical curse placed on someone because of a feud between families or kingdoms.
A strange scientific experiment gone wrong. Scientists might be messing with genetic or energy manipulation and something unexpected occurs. Like they were trying to create a new form of human evolution but instead turned a person into a baby girl because of a miscalculation in the experiment's variables.
I'm not sure if this is a magical or just a very strange story. Maybe it could be about a friend who has some sort of magical power and accidentally turns another into a girl. It could also be a metaphorical story, like the friend's influence makes the person feel or act like a girl in some ways.
In some turned into a girl stories, it could be due to a curse. For example, if someone offended an old witch or a powerful spirit, they might be cursed to change into a girl. Also, there could be a strange natural phenomenon. Like in a place full of strange energy, a person could be transformed into a girl just by being there for a while.
Maybe the friend has a scientific experiment gone wrong. They were working on some kind of gender - changing device or formula. And instead of testing it on a proper subject, they accidentally use it on their friend. As a result, the friend who was a boy is now a girl. This would cause a huge upheaval in the person's life, from relationships with family and other friends to self - identity issues.
Magic gone wrong is a common reason. Just like a magician who miscalculates a spell and accidentally turns someone into a baby boy. Another reason could be a curse. For example, if someone angers an evil witch, she might put a curse on that person to turn them into a baby boy as a form of punishment.
Magic is a common reason. Maybe a mischievous fairy or a clumsy wizard cast a spell wrongly. Another reason could be a strange scientific experiment gone wrong, like some sort of radiation or chemical reaction that alters the body's form. Or it could be a curse from an ancient artifact. If someone disturbs an old tomb and takes an enchanted item, it might put a curse on them and turn them into a little girl.
One day, a boy named Tom had a very strange friend named Lily. Lily was into magic and potions. One evening, Lily accidentally spilled a strange liquid on Tom. To their astonishment, Tom started to transform. His features became more feminine, his voice grew higher. He was slowly turning into a girl. At first, Tom was horrified, but as the transformation completed, he started to see the world from a different perspective. He realized that his friend Lily was really sorry and they both had to figure out how to reverse this strange magic.
One possible reason could be a magical curse. Maybe she offended a powerful sea witch who then turned her into a mermaid as punishment. Another reason could be a strange accident involving some sort of enchanted water. For example, if she drank from a forbidden pool in an old, magical forest that was somehow connected to the sea. Or it could be a result of a selfless act. Perhaps she saved a mermaid's life and in return, the mermaid magic transformed her into one of their kind to join their underwater world.
Magic could be one reason. Like in a fairy - tale world, a magic spell might be cast on the boy. Another reason could be a scientific experiment gone wrong. Maybe some sort of genetic - altering experiment that was supposed to do something else but instead turned the boy into a girl.