How much alcohol? Was he blowing a .02 and there was evidence he used cocaine/xanax in the past 30 days? It's a lot different than if he was illegaly drunk and actively speedballing at the scene of the crime.
That's the bitch too. Granted, getting caught once and then finding yourself in the same situation (worse) makes things worse, as that induces bias when looking at testing results.
Most likely, such tests actually came back with sufficient evidence to accurately demonstrate he was under the influence at the time of the accident.
However, it's also highly, highly possible what showed up in the test wasn't nearly conclusive enough to pinpoint down to the hours leading up to the drug test, but likely demonstrated "this individual must have consumed said drugs sometime in the past three days" (give or take the number of days, not entirely sure on how long after the fact cocaine and xanax can be detected in your body following consumption). But like I said, a guy with a record, testing positively, in an accident - slam dunk case, especially since someone in the other vehicle died.
But shit like this is not always clean cut... he might have be high last night but sober that day... hell, he could have been sober, and he might still have caused the accident too. There are actually a few ways this could go, and no one should be so naive as to think that the legal system produces the correct outcome for a case like this every single time. A lot of bias gets introduced into legal proceedings, no matter how much we try to curb it.