If the gain to her was high enough, and it wouldn't be possible to get that gain (or something close enough to it) in a less harmful way to others, it could be OK. It would be a complicated question with no easy answer, a case in which I don't trust myself to use raw consequentialism because I don't know how to evaluate the real harm done by destroying a Stradivarius, both because I'm not enough of a music fan and because integrating the lost on all current and future humans is beyond my skill. So for high enough values of her gain I'ld be like "are you really sure you can't give her the gain without that destruction ? and if so, well, I don't know".