Sticky situation, man!

Well, congratulations are due to her for at least being able to kick one habit. Like anyone with an addiction you can switch addictions when you come clean from one... I don't want to overgeneralize, but it's pretty frequent that recovering alcoholics may take up smoking as another habit - after all, smoking kills you more slowly than alcoholism can!

It doesn't sound like a very good situation for YOU to be in though, being dragged back into their issues - even if you're going to a birthday for them. Don't do something to make someone else happy, do what makes YOU happy (as long as it's not a danger to yourself or others )

I think perhaps a happy medium would be to send a card for them, and apologize for being unable to attend their birthday but that you wish them well with the rest of their recovery journey.

We can't judge another person's issues because we're not them. Maybe in their case although they're abusing another substance they feel like they've got that better in check than they ever had when dealing with alcohol. Although seeing someone switch addictions is HARD to deal with -- very sad. (I am a hypocrite though since I've got a few addictive tendencies at any given time. But mine aren't exactly life endangering).

If YOU don't think it's sobriety and can't be comfortable with their switching addictions, then I'd advise not going. But I probably would send a card, just to "keep the peace" so to speak.