You are asking a question about what is commonly referred to as "statutory rape," but which Washington state calls "rape of a child (in the first, second, or third degrees, depending upon the age difference and age of the victim). This is governed by RCW 9A.44.079, which reads:
"(1) A person is guilty of rape of a child in the third degree when the person has sexual intercourse with another who is at least fourteen years old but less than sixteen years old and not married to the perpetrator and the perpetrator is at least forty-eight months older than the victim.
(2) Rape of a child in the third degree is a class C felony. "
Here, it sounds like her boyfriend is not 48 months or more older, so it would not be criminal.
This does not constitute legal advice. You should consult an attorney who can assess the facts of your case and discuss your legal rights and obligations.
A consensual, sexual relationship between your daughter and this person is not a crime in Washington. In order to commit the crime of Child Molestation III and/or Rape of a Child III (the crimes that would potentially apply to the situation that you're describing), the individual she's dating must be at least 4 years older than she is. Since he is less than two years older than she is, he cannot be guilty of either of these crimes even if he and your daughter have a sexual relationship.
While willing sex between the 15-year-old and 17-year-old may not result in criminal law charges against either of them, they or someone else may still be charged with some criminal law violation. For example, apparently it is not uncommon nowadays for people, young and old, to take pictures and videos of everything they do. It is unlawful to take pictures or even to have possession of pictures of person under 18 doing certain things.
Moreover, the parents of the 15-year-old have the legal authority to direct her lawful actions and control who she sees. A court may very well grant the parents' petition to prohibit the 17-year-old from having any contact with the 15-year-old.
The states and Canada bordering WA have different ages of sexual consent. If they cross the border of WA, they may be violating the laws of the other places.
If you believe the 17-year-old is exposing your daughter to harmful activities such as drinking, illegal drugs, you should review with your attorney your options in exercising your parental rights.
Nothing else to add that the other 3 attorneys haven't included. NOTE: make sure these youngers are informed of the possible consequences of there actions.