It is so hard to draw these boundaries.
Bwahaha! Sorry, it's funny only because I spent a lot of years with a therapist trying to learn what a boundary was, forget how to set and defend one. If you can find the boundary, you're halfway there - no, you're well over halfway there! So good on ya!
It's perfectly normal to wonder if she's getting anything out of visiting her friend. It would be similar to wondering if someone was getting anything out of, say, a tour of a local prison or a movie you found to be very emotionally moving. Wondering is okay. Trying to change the person or getting into big arguments over it is not okay.
If you had a different relationship, I would say ask her what she thinks of her friend's recovery/rehab. Not if she's getting anything from it, just what she thinks in a curious tone. I'm not sure how strained your relationship is with her or if you think you could maintain a neutral tone in asking.
The boundary is that her decisions are hers. You can inquire into her thoughts on subjects, but that's where it ends. I'm trying to think of a good analogy, but I haven't had my coffee yet, so nothing's coming to me.