There is no objection to associating with the People of the Book socially. However, it is better to avoid them.
Question: What is the criterion for defining the limits of social relations with the People of the Book?
Answer: There is no objection to associating with them socially, while observing Islamic laws and morals.[1] As regards the people of the Book (i.e. the Jews and the Christians) who do not accept the Prophethood of Prophet Muhammad bin Abdullah (Peace be upon him and his progeny), they are commonly considered najis, but it is not improbable that they are Pak. However, it is better to avoid them.[2]
1. Ayatollah Seyyed Ali Khamenei, Practical Laws of Islam, Q315.
2. Ayatollah Seyyed Ali Sistani, Islamic Laws, Najis things, Kafir, 107.