Lampo,
First, was Jesus referring to moon worshipers in the verse you stated? No. He was speaking of someone who shares yuor faith. And the point was that fellow believers are to help keep each other on track.
There are other verses that say we are not to drag non-believers into disputes between believers. But rather we should settle our disputes within the body of believers.
That being said…
I guess the answer depends partly on whether you consider this person a member of the body of believers, or not. If so, and if your friend were Baptist, asking your priest to explain to him the “truth” about baptism is a waste of time.
The bottom line is that this friend is interfering with your right to raise your child. He has no right to interfere unless your child is in some type of danger.
The correct “church” to go to is his pastor. Not to tell him they are wrong, but to explain that he is teaching something that is contrary to your faith.
That should take care of the problem. If the bad behavior continues, whether because the pastor thinks it is OK, or the friend doesn’t listen to his pastor, you need to end all contact with that “friend”.
1 Cor 5:11 But actually, I (Paul) wrote to you not to associate with any so-called brother if he is an immoral person, or covetous, or an idolater, or a reviler, or a drunkard, or a swindler–not even to eat with such a one.
12 For what have I to do with judging outsiders? Do you not judge those who are within the church?
13 But those who are outside, God judges. REMOVE THE WICKED MAN FROM AMONG YOURSELVES.
Of course there are variables that effect how to respond. What I have stated is for someone who is definitely undermining your parental authority. It would not be the same for someone who was asked by your child what his church believes - Not fair to ask him to lie. You simply need to explain why your church does not agree with the teaching of the other church.