"She couldn't make it" sounds like a good answer (thanks hello-kitty).
If asked why, repeat the phrase "she couldn't make it".
Personally, I never felt shame when my son was in detox or rehab...at least he was trying to do something about his problem.
But I get very annoyed when people pursue questions that are obviously not about to be answered. I share selectively with people I trust who understand, not on demand.
Good luck, let us know how it goes.
Hugs