How exactly does alcohol consumption damage dendrites? I'm not talking about over a long period of usage, but just one instance.
Strong evidence suggests that ethanol interferes directly with thiamine uptake in the gastrointestinal tract. Ethanol also disrupts thiamine storage in the liver and the transformation of thiamine into its active form. The role of alcohol consumption in the development of WKS has been experimentally confirmed through studies in which rats were subjected to alcohol exposure and lower levels thiamine through a low-thiamine diet. In particular, studies have demonstrated that clinical signs of the neurological problems that result from thiamine deficiency develop faster in rats that have received alcohol and were also deficient in thiamine than rats who did not receive alcohol. In another study, it was found that rats that were chronically fed alcohol had significantly lower liver thiamine stores than control rats. This provides an explanation for why alcoholics with liver cirrhosis have a higher incidence of both thiamine deficiency and WKS.
 Todd, K. G., Hazell, A. S., & Butterworth, R. F. (1999). Alcohol thiamine interactions: an update on the pathogenesis of wernicke encephalopathy. Addiction Biology, 4, 261-272.
 He, X., Sullivan, E. V., Stankovic, R. K., Harper, C. G., & Pfefferbaum, A. (2007). Interaction of thiamine deficiency and voluntary alcohol consumption disrupts rat corpus collosum ultrastructure. Neuropsychopharmacology, 32, 2207-2216.
but this is of course over a long period.
for just one instance, there's no scientifical evidence AFAIK