Talk with her about it, preferably when you are
not in a sexual situation, but are otherwise comfortable. Figure out what she feels about it. Why does she feel like she 'shouldn't be using them'? Is it fear that she'll become numbed to regular old sex? Is it taboos against pleasuring yourself? Maybe understanding and discussing it will help her get past some things.
Hey, at least she is lucky that she has a guy who cares about her pleasure.
