What is the ruling on interacting with Christian female colleagues at university, including returning their greetings, initiating greetings with them, eating their food (both prepared and unprepared), cooperating on studies, and congratulating them on their holidays? And does the Hadith, "Do not initiate greeting Jews and Christians with salaam..." include all Christians in our time?

It is permissible to initiate a greeting with a non-believer, other than the greeting of peace (Salam), for an overriding benefit or to avert harm. It is obligatory to return their greeting by saying "Wa Alaikum" or "Wa Alaikumus Salam" if they state it explicitly. It is obligatory to narrow the path for them. It is not permissible to congratulate disbelievers on their religious holidays. It is permissible to eat the food of the People of the Book if it is not intrinsically forbidden, and their slaughtered animals are lawful if slaughtered according to Islamic law. Islamic law permits treating disbelievers kindly and showing them benevolence, as long as they do not fight Muslims or expel them from their homes.