A friend from Australia asked:

     His Chabad minyan is always looking for more men to be sure there will be the required ten Jewish men… so, he invited the "boss" of the local Jews for yushka to come pray with them, but on the condition that he not say anything about his beliefs. One of Chabad rabbis flipped out and insisted that he not be allowed to come in. The friend asked me what I thought about it, especially since Chabad is so big on outreach to every Jew.

My response:

     My take on it is, we should try to help the guy without endangering the Community, and if the community sees him in the shul, they will come to assume that he is an upstanding member of the community, which he is not! The specific law might be, not to put a stumbling block before the blind.

     I asked a posek (Jewish law decider) and he said that if he is not coming in for missionary purposes, he can enter a shul so they can try to bring him to repentance. And if he does teshuva (stops his idolatry) he can even be counted in a minyan. 

     So actually, the rabbi was right for complaining: i.e., since the guy cannot be counted in the minyan until he repents from his idolatry, there is no reason to invite him in for prayer. 

    Definitely try to help him, but not in a way that will allow the idolatry to spread.