To be drunk and think that in that state one’s capacity to reason is not diminished is an example of how ones capacity to reason IS diminished! When one is drunk, one’s reflexes are considerably diminished as well as one’s judgement. This is not a matter of opinion. Ask any police officer who has had to deal with drunk drivers. Your friends are in denial.
If you want more information, check with MADD, Mothers Against Drunk Driving: http://www.madd.org/home/
Fr. Vincent Serpa, O.P.