It could be a shy Hufflepuff student who has been observing Harry for a long time. This student might have some special powers or knowledge that they keep hidden. They become Harry's secret friend because they are afraid of being bullied or judged if their abilities are known.
Yes, it is ethical to keep it a secret. Everyone has the right to privacy, and your friend's nude stories are clearly private. By keeping it a secret, you are respecting their dignity and rights.
Perhaps the 'harry x secret friend fanfic' is centered around a secret friend who is from a different magical community. This friend could introduce Harry to new types of magic and magical creatures. The plot might involve them working together to solve a mystery that threatens both their communities. For instance, a dark wizard is stealing magical artifacts from both worlds. Harry and his secret friend have to combine their skills and knowledge to stop this villain.
There are multiple consequences. Firstly, it will destroy the friendship. Your best friend will feel betrayed and hurt deeply. Secondly, it can lead to the breakdown of the marriage. The wife may also feel guilty and conflicted. Moreover, your reputation will be severely damaged. People will see you as untrustworthy and immoral.
No, it is completely unethical. Having sexual relations with your best friend's wife violates the trust and loyalty in a friendship. It can cause irreparable damage to the friendship and harm many people, including your best friend, his wife, and potentially families involved.
It could be a story about a person who has an unseen friend, like a guardian angel. Maybe this secret friend helps the protagonist in various ways, such as protecting them from danger or giving them good advice at crucial moments.