Other teachings in buddhism that are helpful in overcoming addictions are the 4 true realities of the noble ones and the relation between craving (tanha: literally- thirst) and clinging and how this results in suffering and causes us to become people we don't want to be. The first true reality is that distress, suffering etc exists. This first reality of life is to be comprehended and known in all its various aspects. The second true reality is that craving/thirst is the cause of distress, and this reality is to be abandoned, i.e. craving should be abandoned. The third true reality is that suffering and distress can end (through the abandoning of craving). The fourth true reality is that there is a path to the abandoning of craving and the ending of distress, the noble eightfold path which is right view, right resolve, right speech, right action, right livelihood, right effort, right mindfulness, and right composure/mental unification.
The other teaching (which is encompassed by the four true realities) is that craving leads to clinging and this leads to us to become or be people we don't want to be in our lives (and in the teachings on rebirth leads to future existences and thus more suffering). What got me to finally quit smoking cigarettes were the Buddha's teachings on craving and clinging and how it just causes more stress and harm in the long term and it's true. We crave heroin or cigarettes or booze or whatever a shitload and we get attached to the objects of our cravings, we cling to cigarettes always needing to have a pack with us, or we cling to heroin and have to pick up dope everyday or always have it on us so that we can be ready to slam whenever another craving arises and due to this craving and clinging we become addicts and we suffer. It's only through not giving in to those cravings, by not clinging that the cravings will go away. So when we first quit using the things we're addicted to we still have cravings, but we aren't clinging anymore, we let go and stopped that and because of that the craving slowly loses its power and eventually goes away entirely. In order to accomplish this, in order to stop clinging and craving and being stuff we don't want to be we have to have the right view that we can accomplish our goal, that there is a cause that keeps us addicted, craving, and that we can abandon that cause and that we have a way to do it. Then we need to have right resolve to follow through and we have to have the right intentions and thoughts along the way to carry out that resolve. Then we have to avoid talking about our problem as if it's not a problem and we have to speak the truth, that addiction is a problem and it's a f#cking annoying one, this is our right speech. Then we have to take the right actions and avoid hanging out with people that will keep us in our addiction and we have to avoid picking up more drugs or smokes or whatever. Then we have to make sure we aren't selling drugs because that would not be a right livelihood to have if one is trying to give up an addiction. Then we have to put forth right or proper effort, we have to strive and resist those cravings and constantly reflect about the drawbacks of our addictions and avoid the things that would be harmful and cause us to fall back into addiction. The we have to remember all these things and we have to be aware of when craving arises and when it goes away and as we attend in this way one can see with greater clarity the dukkha that addiction and craving brings but one can also see that craving arises and goes away and this brings us hope that one day we'll be totally free from our addiction, this is our right mindfulness. Finally, as we remain mindful and follow all the other factors of the path, gladness arises and we become focused and our mind doesn't jump all around anymore at craving and clinging, the mind becomes more tranquil than it was before we started on this path and this works to strengthen all the other factors, this is right composure/mental unification. Addiction is like the microcosm of the whole buddhist path, if one can use the eightfold path to overcome a strong addiction then one can see how it can work to end all craving and distress and lead to the highest happiness possible, to the unexcelled sublime state of peace and total freedom. So I would definitely encourage your friend to use this path to overcome his addiction because it will be for his longterm welfare and happiness.
"I don't envision a single thing that, when developed & cultivated, leads to such great benefit as the mind. The mind, when developed & cultivated, leads to great benefit."
"I don't envision a single thing that, when undeveloped & uncultivated, brings about such suffering & stress as the mind. The mind, when undeveloped & uncultivated, brings about suffering & stress."