[Please do not quote]
For what it's worth, that sucks and it sounds like there wasn't anything you could do. In my experience with ... somewhat similar instances of people with severe substance abuse, you can't save them unless they want to save themselves. In the event they really, really want to change, that's one thing, and it can happen.... However, I think you know if they just don't / won't ... that's something else. I think it's important to say it wasn't your fault, even if it sounds odd, because you know it's not your fault. The issue may be if you feel it's not your fault rather than if you know it isn't. Feeling and knowing can be different. At least, I find that's common.
If it helps, I've had 2 (3 if you count online) friends come back totally from addiction, but a ... large number that have not. So, I can relate, and it is hard. Point being, you aren't alone in feeling this way.
It sounds like you gave this person a chance, which is nice of you. It's a shame she has her issues impacting her to the point it hits her job into fired territory. If someone is day drinking, or hung over that bad, then at that point, professional help time.... If they won't, and they don't want to, well ... that's a choice they make with bad odds of a good outcome: refusing help.
Entirely understandable to feel as you do.