Is the relationship between a girl and a younger man, based on guidance and mentorship—as if she were his elder sister—considered forbidden and must be severed, especially since she ensures there is no temptation from his side?

The relationship between boys and girls is usually not free from things that are not permissible by Islamic law. The safest approach is to guide the young man to righteous mentors and counselors, as the Prophet, peace and blessings be upon him, said: "I have not left behind me any trial more harmful to men than women." If the relationship were limited to advice and guidance within Islamic regulations, such as avoiding private seclusion (khalwa), unlawful gazing, speaking in a seductive manner, physical contact, ensuring modesty, and adhering to the prescribed hijab, then there would be no prohibition. However, achieving this is highly unlikely. It is not permissible to treat him like a full brother, because he is a stranger to you, and your interaction with him is restricted by Islamic regulations.