When discussing the need for Christians to consider very closely who they choose to marry, it is rather common to look at 2 Corinthians 6:14. While some balk at the use of that passage, it most certainly has application to that relationship. 
Furthermore, Christians should reflect on this passage when it comes to all choices they make regarding close friendships. Read it.
“Do not be unequally yoked together with unbelievers.” What difference does that make? “For what fellowship has righteousness with lawlessness?” (How would you answer that?) “And what communion has light with darkness?” (How would you answer that?) “And what accord has Christ with Belial?” (How would you answer that?) “Or what part has a believer with an unbeliever?” (How would you answer that?) “And what agreement has the temple of God with idols?” (How would you answer that?) God warns every Christian to guard against close associations with those outside of Christ.