Try to take their car keys away if they insist on driving. Your friend may resent it, but if they're too impaired to listen to reason, you must take action. Call a taxi and have your friend taken home. Pay for the cab yourself. Your friend can't object to a free ride home.
Impairment begins with the first drink.
None of these remedies will decrease the effect of alcohol. Even caffeine, a stimulant in coffee and tea, which is supposed to help the drowsy driver stay alert, cannot make an impaired driver sober. Alcohol isn't digested like food, it is absorbed directly into the bloodstream. What this means is that alcohol begins to numb the brain and body almost immediately after it's consumed. Alcohol is a depressant.