This problem, just like alcoholism, or drug addiction, is about standing in the truth. Just because his parents refuse to acknowledge this, doesn't mean that it didn't happen. It did, and it did harm someone.
Getting hung up in getting them to do what is right is only going to add to the frustrations of this event. More then likely, this is not the first time these parents refused to acknowledged that they were wrong, or had not done a good job. Interestly enough, thru their denial they are continueing this pattern.
Move on. You can't change someone else, only ourselves. And here is where your power lies, they can't change the reality of the fact that it happened!
Focus your time and energies in changing the patterns of behavior that this act has caused. Fear? Lack of Trust? Whatever they are, staying in them only continues to give the abuser the power. Take it back!
God can and will heal these wounds, if first you ask.
Peace to you and your husband,