How should a new Christian interact with their friends who are unsaved—especially unmarried friends of the opposite sex?
This is a question best left exclusively to the Word of God. Anyone seeking to know how to act among unsaved friend should read this passage, and spend time considering all that it says.
Therefore, prepare your minds for action; be self-controlled; set your hope fully on the grace to be given you when Jesus Christ is revealed. As obedient children, do not conform to the evil desires you had when you lived in ignorance. But just as he who called you is holy, so be holy in all you do; for it is written: "Be holy, because I am holy." Since you call on a Father who judges each man's work impartially, live your lives as strangers here in reverent fear.
1 Peter 1:13-17 (NIV)