Well I don't know about your friend being jailed simply for a .058, that's just a DWAI and easily taken care of with a summons..... must have been other issues during that arrest?
There's a lot of confusion about "legal limits".....
The difference between the DUI .08 and the DWAI .05 lies in the burden of proof and penalties.
DUI: You don't even have to show bad driving (speeding, failure to signal, accident) to be convicted - although it helps. The law simply states "operates a motor vehicle with .08 Blood Alcohol Content (BAC)". A person can simply put the keys in the ignition while sitting in the passenger seat and that constitutes "operating" the car. Lots of case law on this gray area of the vague definition of "operating" but it has been upheld often enough. The State has determined that no one as a matter of scientific principal can safely operate a car with that BAC level. Conviction carries 3 month license suspension first offense, 1 yr second offense. Up to 30 days in Jail, court costs and fines.
DWAI: This being the "lesser included offense" to DUI, it's used many times as a plea deal for a DUI charge. But it is also an offense in itself. Generally, the courts won't uphold a DWAI charge without an included "probable cause" charge of speeding, weaving, etc, that shows bad driving. Basically, this is where your issue lies - A person could possibly operate a car safely with a BAC of .05 to .079, but without probable cause to show bad driving, the roadsides, portable breath test and other tests would be without merit. The case and it's prosecution lies in evidence that shows the impairment caused the bad driving. Conviction carries no jail term, and license suspension is not likely, generally just lots of points off your license and fines.
But simply stated, the law states that the .08 BAC level is the "limit" where any question as to a person's ability to drive safely is no longer subject to arguement. If you're caught driving with a .08 or higher, you're in violation of the law - period. Any BAC between .05 and .079 is argueable and requires a bit more proof to show "impairment" which is in essence the violation - alcohol caused bad driving below the legally defined "limit".
Does that make sense?