If your friend was at or over the legal limit an in operation of a motorized vehicle, then my suggestion is to save your money on the lawyer, pay the fine, learn from this lesson.
As crappy as it may seem because "He was just backing down the boat trailer" the law says you can't operate a motorized vehicle. I'm not trying to be a dick, I got a DUI for a similar thing (parking the boat trailer) and I spent 5k for a lawyer. All I got for that 5k was never having to go to court, not having to do community service in Banning picking up trash on the fwy for a total of 80 hours to haivng to do 100 hours of community service at the Goodwill in Huntington Beach.
HBjet