The way to help an alcoholic is to set limits and boundaries with that person so that you are not enabling them or letting them take advantage of you.  The way to do this is by communicating with them while they are sober exactly what is not acceptable behavior for you, and what the consequences of that behavior will be.

If you want to know how to help an alcoholic then you need to get clear on what these boundaries are first.  You might need help to figure that out and you can get that help and advice from an Al-Anon meeting.