So long as the "friendship" does not include or cause you to violate your walk with Christ, I see no problem. I have a dear friend who I believe to be saved, yet one day he veered off into homosexual behavior. I did not shun him, I maintained my usual behavior with him except that I used every opportunity Christ provided to remind him of his duty and obligations to Christ as a professing Christian. In time he closed off contact with me. I continue praying for his repentance and restoration to a sound Christian walk.