There are not very many people in this world who find it easy to initiate such a conversation. However, John and Andy have worked together for several years. During that time, John has developed a respect for Andy's work, and it appears that they have a close working relationship. Andy is extremely fortunate to have a friend like John. John may be the best person to talk with Andy, and he should do so.
Friends can have an impact when they show genuine concern.
Friends don't stand quietly by and let friends abuse themselves and their future with harmful substances, without expressing their concerns. Certainly, there are important aspects of privacy and freedom to consider, but an honest attempt should be made to communicate. Often, substance abuse is a symptom of low self-esteem. This may not be the case in Andy's situation, but if it is, it will be encouraging to Andy to find out that John values him enough to discuss the problem with him. John's concerns are genuine and sincere.