She might be embarressed, I remember doing this once because I was embarresed. Is the bin in the bathroom closed? Our bin was open and I didn't like the idea of everyone knowing I had my period. Yes it is disgusting and yes not very hygenic (And I am a super clean freak) but at the time I couldn't put two and two together, probably naive I guess. In the end I put a bin in my room because I preferred that than using a bin that everyone could see my business iykwim?

Maybe talk to her about why she's doing it iykwim? If she's embarressed this might only make the problem worse.

*hugs*
Cailin