Long term abuse of alcohol can adversely affect a person who stutters because it alters various neurotransmitters in the brain, which for a person who stutters, can degrade fluency even more over time. I find when I’ve had a heavy drinking night, I might be fluent temporarily whilst intoxicated, but the next day is absolute hell got my speech fluency. If your friend has been abusing alcohol for long periods, it’s possible he’s suffering from severe dopamine and GABA dysregulation / dampening. Stutterers inherently have dysregulation of dopamine in the brain, which is potentially linked to dopamine over-activity in speech motor controls, causing dysfluency. He needs to get his drinking under control, and seek out help to address his stutter. It’s not going to be an easy journey.