When my alcoholic first got sober, I kept track, and would congratulate him and even ask that my family congratulate him too - I was just so proud of him!
But then he relapsed, and I imagined his humiliation if someone would have congratulated him on an 'anniversary' that was no longer applicable. He is already really hard on himself and I can only imagine that would make it so much worse. Additionally, my sister and her boyfriend went to rehab for narcotics, and when they had 6 months sober I sent a gift letting them know how proud I was. However, I found out later that the boyfriend had relapsed (and was getting clean again) and he felt guilty accepting my gift.
Now I don't make a big deal out of it - like the woman above, I just say "I'm proud of you" every once in awhile. And I probably won't say anything again to my sister's boyfriend....while I hope the best for him, its not my business, and I'd never want to make him feel bad again.