DUI law states that you are in violation if you are UNDER THE INFLUENCE of drugs while driving. It can be alcohol, maryj, or prescription medicatin; doesn't matter. The .08 level is written into the law for alcohol only and it basically states that IF you blow .08 or above, you are presumed to be under the influence. THe police have a much lower burden of proof. However, you can easily get arrested at any BAC provided your driving behavior indicated that you were 'under the influence.' Some jurisdictions make the arrest on a 'low blow' if you were involved in an accident or the driving is criminal in nature (wreckles driving, criminal speeding (100 on the fwy as an example), or anything esle that is criminal in your state.
Also, I do not know your age but we ALWAYS arrest for any BAC level if the person is under 21. By law, they cannot have ANY alcohol in their system by driving. So, if your friend was 20 and blew a .01, that's enough. There is usually a separate law for under-age drinking/driving.
Keep in mind, police cite all offenses in this case as the court will drop the majority of them. So, you can get 2 charges of DUI in one arrest. The court will plea out the second charge.
Hope it helps.
My best advice......tell your friend not to drink and drive. I've seen LOTS of dead teenagers that think they're great drivers after a beer or two. LOTS. Watched one kid die in front of me.....he was alive when I got to the crash scene and dead when he left in the abulance. Then I had to tell his father (in person) that his son died.