I was a closet drinker, too, and didn't tell any of my friends, but I'm not sure any of them were as close to me as this friend of yours - it sounds like she is a very special person to you. As hard as it seems to tell her, think how much easier your heart will be once you do. Right now it sounds like you feel a bit of stress on the relationship because you can't be open with her, but telling her will change that stress into an additional source of support. You may find that having her know "keeps you honest" and prevents relapsing! Whatever you decide, our thoughts and best wishes are with you!