Can you help with my friends "loaded question"?

A friend, I have known for many years, asked me a personal question. I was hesitant in giving him my answer, simply because of our long friendship. Here is the situation: my friend and his wife have been married for many years. He tells me that last year, his wife suddenly decides to quit having sex with him. I could tell this was upsetting him physically and emotionally. He asks me numerous questions, in which I could not answer. I want to help my friend, but since I have never been in this situation, my answers were few and far between. So, this is why I ask this question of you. Question: what would make an older woman suddenly start denying sex to her husband of many years?