Even if the police tell you differently, NO ONE is required to speak with the police; witnesses, perpetrators or victims, and EVERYONE has the right to counsel. Your Miranda rights always apply, but the police are not required to advise you of them until they arrest you.
If she's still there, her silence may cause the police to be suspicious of her possible involvement, but that is their problem. She may even be arrested on the basis of their suspicions, but they still have to prove a case against her. If she is only a witness, it won't go very far.
If your wife is *-only-* a witness, she is free to leave before the police arrive.
This is not a crime. You don't have to tell the police that she was present. You don't have to tell them anything and neither does she. If any other witnesses are still present, point them out to the police.
If she leaves the scene and starts contacting your attorney, this can be a real time-saver for you. The attorney will already have her version of the story and be prepared when you meet.
If there is still danger, YOU can leave the scene as well. I strongly advise calling the police yourself to report the assault in any event.
You want YOUR voice on that 911 tape. No details; just your name, "he/they attacked me/us", where you and they are at the moment, whether anyone needs medical attention and that you are no threat to the police when they arrive.
While you're doing that, she's in a taxi and calling your lawyer on the way home. If your vehicle is not involved in ANY way, she can drive it home and prevent it from being impounded "because it might be evidence".
It will be up to the police to prove that it is evidence. If you weren't in or near your vehicle during the attack, it's merely some of your property in the vicinity. That does not make it evidence or automatically subject you to towing/impound fees.
Police: "Where's your car?"
You: "I want my attorney and I do not consent to any searches."
Police: "Witnesses said you were with a woman. Where is she?"
You: "I want my attorney and I do not consent to any searches."
Police: "One witness said you called her 'honey', told her to call your attorney and go home. Who was she and where did she go?"
You: "I want my attorney and I do not consent to any searches."If you see a pattern here, you're getting the idea. Heh.
The problem I see here is despite all this advice, when the time comes, under the stress of the moment we will not be able to remain silent.