Well, you just gave yourself some of the best advice possible - consult an attorney.
Keeping yourt mouth shut when the officer came around was a good move as well. As far as liability goes on your part, well, there is a host of issues that can be raised here. I'm only versed in Canadian law, so I'll tell you what could occur from our perspective:
1)Obstruction of justice is a possibility here, though as far as I can tell, based on what you have told us, is that this charge could be difficult to stick. You are not obligated to say anything about the occurrence without legal representation present, however, you ARE required to tell the officers everything you know about the accident once your counsel is present. In actuality, I can most assuredly tell you, that a lawyer would advise in your best interest to tell the police everything you know and what happened. Not to put your friendship with your friend on the line but you really need to think about #1 on this. You gain nothing by lying to the police - you didn't commit the crime, you were simply in the passenger seat.
2) This is where things get a bit more tricky - Though you are right, saying nothing is your right and that omitting to act isn't a crime, when the police start asking questions you have to start answering, otherwise, this goes back to #1. Saying nothing in this case only means you are not obligated to pick up the phone and call the police after the accident.
Really, the only offence as far as I can tell that can even be remotely laid on you is obstruction of justice. You weren't a party to the offence as you in no way aided in the commission of the offence (unless it was your car and your friend was drunk and you knowingly gave him the keys) and you aren't an accessory after the fact as you didn't aid or abet him with intent of getting him off the hook. You simply kept your mouth shut.personally, if I were you and knew then what I know now, I would of just told the police everything. Although it is generally a bad idea to spill your guts to police without legal counsel present, as I said before, most lawyers would simply tell you to co-operate. You gain nothing by bullshitting the police and open up a whole new can of worms.
In all probability though this case won't become much to talk about. All that was done was property damage (and I'm assuming the city's) and a little bit of false paperwork. Your friend is most probably looking at a fine and possible civil action if the trees were owned by a citizen or private company. Either way, if I were the officer in charge of the case, I certainly wouldn't waste my time and resources trying to hit you with an indictable offence (felony in US) that probably wouldn't hold in court.
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