They will call him a liar even if they have no proof. A prophet's message rests solely on his truthfulness. If he were ever to lie, even to save his life, then this would prove that he is capable of lying and that he has established for himself guidelines under which it is permissible to lie. This would undermine his whole message as no one could then be sure he had not convinced himself that the end justifies the means, and that in order to get them to become decent people he might be willing to fabricate lies against God himself. How much worse to drink oneself into a stupor.
Alcohol is the door to all evils.
Once a person loses control of his faculties he will be capable of anything. He who is willing to drink in such a fashion must realize that he will be accountable for his subsequent actions. Think about the other allegations for a while and you will understand what we mean. Muslims believe that the prophets of God are above such actions. A Muslim believes that when God selects a messenger, He chooses the best of the best.