As an "in the dark" the wife of an alcoholic, I would advise you to be honest with her. I have been married for 20 years and my husband has had a problem for the last five years.
Things have gotten better since I found out the extent of his problems. Before, I was actually contributing to frequent relapses, because I did not understand alcoholism. I think few people do... while I thought he had a drinking problem (ie he would be having too much), I did not understand the power that alcohol had over him. I was condoning/encouraging him to drink in moderation. I did not understand that this was not possible for him.
I enjoyed an occasional glass of wine, but have gladly given that up for his sake. Since I'm not an alcoholic, it really doesn't bother me much. Having an open, honest relationship with him is far more important.