She keeps tweaking

I cannot tell what is happening at my job. One of my co-workers is stealing money daily. She rings things in that are way cheaper than the actual item and then pockets the difference when the customer leaves. I've caught her doing it a few times.
I can't tell on her. She is my best friend that I helped get the job and I know why she is stealing the money. She started using meth around a month ago at a party we went to. She keeps tweaking on weekends. I am afraid she'll start doing it during the week too and really get damaged. I don't want her to get fired but I want her to get help. What do I do?

Name kept secret
Location kept secret