The prior poster is correct that the age of consent in Washington is 16, as long as you are not in a position of trust, such as her school teacher, etc. However, it's unclear whether or not you could be charged with communication with a minor for immoral purposes. If the minor lives in Oregon, the age of consent is 18, and if the man were to give a ride to the minor to Washington for the purposes of having sex it could be a federal crime under the Mann Act, or even possibly the man could be charged in Oregon. These are possible issues, although, unlikely to arise since the family approves of the relationship unless the relationship takes change in the next few months.
As stated, the age of consent is 16. That being said, the man could be charged with communicating with a minor for immoral purposes. If you believe a crime has been committed you could talk with an attorney or contact the police where the crime occurred. They will investigate and make the ultimate charging decision. Many times, the family approval will have little impact on a charging decision.