Well, I’m not a cop, but I did work as a Breathalyzer technician for about 5 years once. Here’s my take.
First, in order to be pulled over, your friend must have committed some sort of vehicular goof that attracted the attention of the cop. So, the minute the cop pulls him over, the cop is already 90% certain he’s going to be issuing a citation for something. He gets within sniffing distance of your friend’s breath and detects alcohol. Now, he’s 100% certain he’s going to issue a citation, and 90% certain he’ll be making an arrest for DUI. All that remains is to gather the necessary corroborative evidence that your friend was too impaired to drive. So, out come the ‘field sobriety tests’ for coordination, balance, etc. The breath test comes last simply, I would say, for convenience and practicality.
When your friend gets to court, the fact that he passed the physical tests should count somewhat in his favor (assuming the cop recorded the results fairly). What matters now is, how good an attorney will your friend have?
Since I don’t know exactly what the DUI laws in your state are, I’m somewhat limited in what I would recommend. Is this his first DUI offense? If not, how recently was his last one? If so, what court/judge is he likely to go before and what is the jurist’s record for DUI sentencing? Will your friend fight the charge? If so, is his attorney someone who ‘specializes’ (to the extent his state does/does not recognize specialties of law)in DUI cases?
What sort of breath test was administered? Was your friend advised of whatever special rights he may have under local law regarding getting his own test, refusing the test, having an attorney present, etc.? If a machine was used, was the officer who administered the test certified to know how to properly operate the machine? Could he have proved it, at the time of the test, by showing his certification? Was a daily log kept of the testing of the machine for proper operation by certified personnel? Was the machine tested against a sample of known concentration immediately after your friend’s test, to determine that it was in proper operation at the time? All these are questions that his attorney may want to raise.