Sometimes as women age they do begin to lose the desire for sexual relationships, this is a very common problem. But not one she has to suffer though if she wants to give it a try. The first issue is 90% of the time this is a simple medical issue. As women age hormonal imbalances are more common. There are many types of things that can be imbalanced here including progesterone. Progesterone is responsible for the libido but can fall quite low.
Fatigue can also cause this, so if she is not getting enough sleep or is over extending herself her libido will suffer. Stress and depression can and will also cause this, as well as a few other medical conditions so when ever a women loses all desire the first step is to speak to her doctor. While she may be a little upset at the thought of seeking medical advice the reality is her health is far more important then her sex drive and therefor she really needs to have this looked at for her own medical wellbeing.
There are medications that her doctor can prescribe that will help level out her hormone levels which is what is likely causing this.
I would suggest having her take a look at the following site with information on this.