Good. I don't know the WA system as well as other states, but courts are becoming more and more proactive about ensuring children have a relationship with both parents, and will change residency where it deems it necessary to do so in order for the child to have those parental relationships upheld.
If the consultant writes into their report that they suspect the mother is coercing the child and deterring her from having a relationship with the father, then the father will have a much better case to argue that the child should live with him and have a period of no contact with the mother while the child settles.
Just make sure he doesn't descend into damning the mother. If he can show the court that he wants the child to have a relationship with both parents, it will improve his position.