Wow. I totally get the logical arguments on both sides, but if I were to judge the situation, I would totally err NOT on the side of rules or dogma, but on the side of compassion for the person.
You don't say whether the group asked her why she took the oxy, whether they offered kind and helpful advice, etc. so it's hard to really know what happened.
Gosh, though, if it were me, and I admitted to taking one unscripted oxy, and a group of people set my sobriety clock back 5 years? I think I'd be so sad and humiliated I'd want to drink.
Based on what you've said, I do think the "punishment" was a little draconian and inhumane. Why would she confess if she weren't, on some level, asking for help, advice, understanding, empathy, compassion or something along those lines?
I hope she at least got THAT in return for seemingly having been treated like Hester Prynne in The Scarlet Letter.