The same thing we do for all unbelievers: witness to them.
I do not view Islam as a "counterfeit," because it doesn't try to replicate Christianity. It is a cult, as every group that misidentifies Jesus Christ is. But we are called to love those who hate and persecute us, to preach to them the Gospel just as we should to any other cult. And like most cults, you're going to find people who "grew up in it." You'll find people who are associated because, well, the people are just so nice. People who were mistreated by "Christians." By Christian parents. By a society they couldn't relate to. But still people who need Christ.
God bless.
(Mat 10:14 KJV) And whosoever shall not receive you, nor hear your words, when ye depart out of that house or city, shake off the dust of your feet.
(Luk 6:22 KJV) Blessed are ye, when men shall hate you, and when they shall separate you [from their company], and shall reproach [you], and cast out your name as evil, for the Son of man's sake.
(2Co 6:15 KJV) And what concord hath Christ with Belial? or what part hath he that believeth with an infidel?
(2Co 6:16 KJV) And what agreement hath the temple of God with idols? for ye are the temple of the living God; as God hath said, I will dwell in them, and walk in [them]; and I will be their God, and they shall be my people.
(2Co 6:17 KJV) Wherefore
come out from among them, and
be ye separate, saith the Lord, and touch not the unclean [thing]; and I will receive you,