If parents on both sides vouch for the kids, the 18 year old in Idaho, the 15 year old in Washington, can these two individuals still have a relationship without it being illegal?

As long as it is not sexual

Depending where u live, once the 15 year old turns 16 it will be legal

Even if parents from both sides are cooperative and let the two be together?
And wouldn't the only way that it would become a problem was if someone 'told on' this couple?