Answer:
Alcohol is alcohol, and depedning on your weight, even just 2 drinks in your system would be enough for you to be considered impaired. Your friend may feel fine, but after several drinks they are not functioning as well as they should, and will still get in trouble with the police if they get pulled over and are breathalized. Tell your friend that you will drive home; if you both have had drinks, find someone else to drive you or call a taxi. It doesn't matter if it was 1 drink or 6, alcohol is alcohol.