Too many variables are unknown to accurately determine how much burnoff this kid had.
You pose a bypothetical and I will return one to you and see what you think about it.
Lets say he had left wherever he was at 15 minutes before he crashed into us. Lets say that right before he left he did four shots with his buddies or did a five foot beerbong.
In this event it is possible that his BAC at the time of the crash was equal to or less than when he blew.
I offer this only to take notice that there is no way to accurately measure such a thing as you attempt to measure without more information.
In other words, you are suggesting the hypothetical that he might have drunk so much liquor so quickly that it hadn't all been absorbed into his bloodstream at the time of the crash?
I suppose that's theoretically possible, though alcohol is absorbed pretty quickly. But unless you believe the guy drank alcohol between the time of the crash and his breathalizer test, he must have consumed enough to get his BAC well above 0.15 in order for him to blow a 0.123 three hours later.
As for there being too many variables, I can't tell you exactly what his BAC was. But for him not to have consumed enough alcohol to put it over 0.15 at the time of the crash, he would have to have weighed about 400lbs (using ; a 400lb male drinking 16 U.S. beers over 3 hours yields .125; 16 U.S. beers over 0 hours yields 0.151). Did the guy weigh 400lbs? You tell me.