In WA, merely having a child does not make a person a legal adult. In WA, a person becomes a legal adult upon turning 18, marriage, or being emancipated by the court. Marriage before 18 requires parental approval or court order.
Legally, the girlfriend is still a child until she meets one of the above conditions and is subject to the legal control of her parents.
Practically, the police is not going to do anything to a 17-year-old who leaves voluntarily her parents' house when there appears to be no immediate danger to the 17-year-old. Of course, unless the relationship with her parents is already terrible, alienating her parents is not a good idea for the 17-year-old.
Alienating your likely in-laws is also no a good idea for you or your son.
She likely needs all the good help she can get from her parents at this time.
You all should sit down and have a thoughtful discussion on what is best for everyone involved at this time.