* Ethical Concerns: The power dynamic between a teacher and a student is inherently unequal. Living together blurs the lines of that relationship even further, creating the potential for exploitation, abuse, or the appearance of impropriety. Regardless of the intentions, it could severely damage the teacher's professional reputation and jeopardize their career.
* Legal Concerns: While the student is legally an adult, depending on the specific circumstances and location, there could be legal ramifications for the teacher. Laws regarding teacher-student relationships vary, but many jurisdictions have policies or statutes that prohibit or severely restrict interactions between teachers and students that could be considered inappropriate or exploitative, even if the student is 18. This could include investigations and potential disciplinary actions by the school district or even criminal charges.
* Potential for Misunderstanding: Even if everything is perfectly above board, the appearance of impropriety alone can lead to negative perceptions from parents, other students, and the community.
In short, while the student is an adult, the relationship between a teacher and a former student is still fraught with potential problems. It's highly inadvisable for both the student and the teacher. The student should seek alternative living arrangements.