Seriously, how did you get to the conclusion that a) she's abusing it and b) that she's doing that without a prescription?
Because when a doctor prescribes Percocet and you go to a pharmacy to get it, you don't accidentally end up with fentanyl instead. That only happens when you buy it illegally from a dealer.
Also, doctors normally don't prescribe percocet or other strong opiates to "take the edge off" (i.e. psychiatric treatment). That's what barbiturates and anxiolytics are for.
If she's got a prescription, then this needs to be headline news because the supply is tainted and will kill people.
More likely, she obtained the pills without a prescription. I won't speak to abusing it since that can be such a loaded word.
That or the pharmacist made a horrible mistake and would be headed to court and possibly jail. Also newsworthy and also a relatively unlikely explanation.
She mentions relapse and years of sobriety, plus fentanyl is a pretty extreme opioid, I think they only give it to people with things like very advanced cancer, so it seems pretty unlikely her prescription was accidentally filled with that.