It's difficult when they are so functional. It's easy to think that reason and logic can get through because they seem normal from time to time. And normal people would notice how destructive they are being. Alcoholics are not normal. They are gripped by something that can't be logically explained or reasoned with.
It's difficult to watch a loved one slowly destroy themselves. But we are powerless over their addiction. What we can do, it take care of ourselves.