Provide them with information to read later. Involve the person in the assessment of their use of alcohol and other drugs. Offer acceptable alternatives and choices. Tell the person the consequences you believe further use will have on their health, life, and relationship with you. Expect lots of excuses and minimization of the problem.
Most people are reluctant to admit that they have an alcohol or other drug problem.
Recognize your own limits. Sometimes you can only help so much. If you have provided a person with an honest appraisal and left them with options for where they can get help, you have done your share. Now it's their responsibility to follow through and get help.