What is the ruling on greeting and interacting with professors and students of law at the faculty, and what is the ruling on not greeting one who is openly committing a sin or abandoning prayer? Does this fall under enjoining good and forbidding evil?

It is recommended for a Muslim to offer greetings of peace (salam) to everyone, whether known or unknown. There is no harm in studying or teaching man-made laws (positive laws) for a legitimate purpose. Offering greetings of peace to those engaged in man-made laws is permissible. If a person is openly committing a sin, then withholding the greeting of peace from him with the intention of admonishment falls under enjoining what is right and forbidding what is wrong. As for one who abandons prayer, he must be advised, and it is legislated to boycott him if it serves as a deterrent; otherwise, there is no need for boycotting, but rather to strive for his rectification.