“How can you prevent alcohol addiction?”

In order to answer this question, it’s important to understand what is “addiction” and where does it come from. As with any behavioral or thinking problem, typically it’s acquire from part biological and part environmental. Using your example, your friend might be predisposed to an alcohol addiction if this disorder runs in their family (this would be the biological component) and even if this is the case, that doesn’t necessarily mean your friend will struggle with alcoholism. If, however, your friend already has this biological predisposition and begins to drink excessively whether socially or as a coping tool, there is an increased chance it may lead to alcohol addiction. That said, the “addiction” is truly about the “relationship” the person has with the thing, whatever the thing is. Sometimes people quit one vice only to transfer their maladaptive relationship to another (gambling, food, shopping, etc.). Alcohol is not necessarily the problem, rather the quantity, frequency, impact it has on the person’s life is what can become problematic. Alcohol is also an interesting vice, so to speak, because it is both a depressant and also tends to be negatively reinforcing. What that means is as follows:
Let’s say your friend drinks to reduce stress. The drinking begins… the stress is decreased (because the person starts feeling the effects of the alcohol), so that’s reinforcing! Next time there’s stress, guess what’s going to happen? The person will do the thing they did that decreased stress (drinking). Now, remember I said alcohol is a depressant? After the euphoria comes the crash. The person starts feeling down in the dumps (depressed). It doesn’t solve the issue, obviously.
Another thing to understand about addiction, is there’s something that gets classically conditioned (meaning “connected) so that the person’s system will recall that alcohol = stress relief, fun, social disinhibition, whatever it is that leads them to drink… so it will become habitual (meaning addictive behavior) to use the thing, alcohol in this case, to achieve desired outcome. I would suggest your friend speak with a professional (licensed therapist) to discuss this if it’s a concern.

Choose a time when your loved one is not drinking and you're both calm and focused. Express your concerns in a caring way. Encourage your loved one to open up about the reasons why they're abusing alcohol. Consider staging a family meeting or an intervention if you'd rather not go it alone.