I suggest pointing out any particular scripture will not suffice. Your mom will find a WT article which discusses it. It won't matter what it says, the fact that there is a WT answer, no matter how weak, will be taken at face value.
Therefore, I suggest a direct attack on the monster itself, the notion that Acts 15 supports the notion of a centralized governing body. The narrative the WT presents is that there was this happy little GB sitting around in Jerusalem who governed matters of doctrine and practice. Indeed, if you read Acts you can almost come up with that notion, Acts is such a sanitized, white-washed church history.
But, we have a completely different picture when we compare what Paul said about that body of old men. In Galatians 1 Paul goes out of his way to point out that he did NOT get his authority from "those who were apostles before I was". In fact, he only met Cephas three years after his Apostleship and did not bother with the others at that time. In fact, he was unknown to those in Judea. This was a GB?
Note how Paul goes on in Galatians 2 to describe his meetings with these folks in Jersusalem, these important Christians. He is very specific, very deliberate that he did not need, seek, want, recognize, thier endorsement. He was letting THEM know what he was doing, not asking permission. "those highly regarded men imparted nothing new to me. On the contrary, when they saw that I had been entrusted with the good news." In fact, he talks about how he resisted Cephas later over the matter of the law.
Galatians, to me, shatters the entire idea of a central governing body in Jerusalem. Paul is saying, rather directly, that the Judean "Christians" were backward and working against Christ in thier adherence to the Law. Acts 15 is not a story of a governing body, but of the Jerusalem congregation coming to terms and accepting the teachings of Paul. It is the settling of a massive dispute between several power bases, not the expression of a single central power base.
Reading the two accounts side by side shows how the WT twists the story to create a pattern of authority which simply did not exist - the opposite existed.