If there are no charges against your friend you are not aiding anyone. The police feel that if you do not fully cooperate with them you "obstruct justice". You do not seem to be doing that. You do not have to speak to the police. But the officer can do what police do, intimidate and threaten. You may want to call and say that you have provided all information. I would advice your friend not to give a statement, at least not without counsel present.
"Similarly, officers may seek consent-based encounters if they are lawfully present in the place where the consensual encounter occurs...... If consent is freely given, it makes no difference that an officer may have approached the person with the hope or expectation of obtaining consent. ...." Kentucky v. King, 563 U.S. ___ 091272 (2011)(internal citation omitted).
Most importantly to your question, the Supreme Court of the United States further said as follows:
When law enforcement officers who are not armed with a warrant knock on a door, they do no more than any private citizen might do. And whether the person who knocks on the door and requests the opportunity to speak is a police officer or a private citizen, the occupant has no obligation to open the door or to speak. (“[H]e may decline to listen to the questions at all and may go on his way”). When the police knock on a door but the occupants choose not to respond or to speak, “the investigation will have reached a conspicuously low point,” and the occupants “will have the kind of warning that even the most elaborate security system cannot provide.” And even if an occupant chooses to open the door and speak with the officers, the occupant need not allow the officers to enter the premises and may refuse to answer any questions at any time."
Kentucky v. King, 563 U.S. ___ 091272, 16 (2011)(internal citations omitted).
Homes have an extraordinarily high privacy requirement and protection under the United States Constitution. However, looking at Kentucky v. King, absent a warrant, arrest, probable case, administrative regulations, and/or some grant of authority from the Commonwealth of Virginia, then I don't find much between a police officer investigating and a citizen investigating based on my reading of the U.S. Supreme Court's decision in Kentucky v. King.
Really though, you should seek counsel and get some advice. You may have missed facts that are relevant. You may need the advice yourself. Lastly, the United States Constitution and its protections can be both rock solid and at the same time sand. So please seek counsel.
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