I would like to help you with your question.
This is an unfortunate situation. If your daughter is unwilling to get the police involved and locks her door out of fear then she is allowing her son to control the entire family. That's a case of the "tail wagging the dog" and it needs to end. What her son needs is tough love. As it is, he is walking all over the family and getting away with it. Does she think her passivity is going to help?!
And his attitude that nothing will happen no matter he does is true based on the fact that the parents sit by and allow him to rule the roost.
I'm typically opposed to extreme measures...but in his case getting thrown in jail overnight might shock the kid and get him to see that there are consequences for poor choices. In the same manner, having the parents be held accountable for their sons behavior might wake them up as well. After all...he is 17 and that means he is underage. Having the police knock on their door and bringing them down to the police station for a little chat might be enough to get them to see that they cannot abandon responsibility for his actions.
I'm sorry you are caught in the middle of this nonsense.
You mentioned counseling wasn't working. Is this family therapy? Or is the son in therapy?
Therapy can only be successful when the individual or individuals want to change. I'm not sure what motivation any of them have for changing...
And yes...if your daughter isn't willing to put some effort into taking back control of the family...about all you can do is to listen and support her feelings.
I'm going to suggest a very good website for you to tell her about:
This is a highly successful parenting philosophy. The website has wonderful parent tips.
Look for the tape:
Hormones on Wheels by Jim Fay.
This is EXCELLENT.
The love and logic program puts parents in the lead...right where they belong.
I await your response.