I'd opt for a gun safe. They are extremely sturdy and designed not to be easily broken into.
Morgan has a great idea about the false panel boxes, or even the "fake products" that hold contents inside, but look like something else on the outside. I think hiding something in a tampon box would usually keep men out, or anything relating to feminine hygeine. (Random thought).
But boxes that are fireproof, like many gun safes, offer added security and strength. Nothing with a padlock.
Out of curiosity, if I may ask, what happens when she confronts her spouse about the missing pills?