Alcohol is the one drug that can be fatal during withdrawal. No, heroin does not produce fatal withdrawal symptoms (they just feel like they are going to die). So to answer your question, it depends on many factors as to whether they can quit on their own without help or medical monitoring. The best way to assess this is to have an addiction professional assess them. A person can quit and not have the severe withdrawal if their use was not too great, even if they did drink every day for many years.
I was consuming a fifth of vodka, wiskey or tecquila a day, every day, when I stopped but I went into in patient treatment and I was professionally assessed for risk from withdrawal before I was admitted to the facility. If I was a great medical risk they would have diverted me to a detox center first, then admitted me.
The other important issue is underlying medical conditions of this person. Especially if they have any cardiac or pulmonary medical conditions. Alcohol withdrawal can be particularly deadly with these conditions present.
Caveat: it is easy to stop. The problem is not starting again. That is the difference between recovery and mere abstinence.