Ooona

Many of the Substance Abuse IOP programs are integrating trauma treatment into their programming. This is coming out of evidence based practices that show a correlation between trauma and the problematic usage of alcohol and drugs.

Either way, I can understand why you are confused and concerned, you dont have an understanding of addiction, obviously havent been through this personally. The best thing you can do is to encourage her be supportive etc. If she begins using again, there will be enough signs and her life will become unmanageable in significant ways, or she just wont show up for work anymore. In the meantime, dont set her up to fail by looking for reasons not to trust her, give her a chance.
She may just surprise you and become the best and most trustworthy employee that store has ever seen!