Any hormone or vitamin she could take (and this is also true with Viagra), she still has to want to have sex, for it to work. If she is turned off from sex with you emotionally, no drug will make her excited. If this is bugging her, she should see her doctor. If it is not bothering her at all, I'd take it as a signal that she might be losing interest in the relationship. Then either work on that (i.e., why she is and what you might do about it), or decide to move on.
I would recommend she talk to her GYN or regular physician if SHE wishes about this.
I am not sure if this is a psychological or a physical issue going on with her.