Addiction to heroin is no joke no matter how she is getting high from it. My addict (she) is also a long term heroin user. Also mixed in numerous other drugs. Which is common.
Im sorry you are dealing with this, but my advice is to just let her be. No one knows better than me that you really cant save fix or change her. I tried for years. Heroin & addiction are running this show not you (same for me). Trust me if she wants to score she will. Heroin addicts are completely driven to keep using no matter what. They are very cunning. You cant stop it. I couldnt stop anything.
Save yourself & get away from her far away. Addiction to heroin is extremely destructive. I have unfortunately witnessed it all to no end first hand.
Im sorry I dont have better news for you.