I don't know you and your whole situation, so I don't know how much help I could be. All I can do is relate my own feelings and experiences.
I love my husband very, very, very, very much, but I wasn't satisfied with our sex life. It wasn't his fault or my fault. It was only a matter of drive. My sexual frustration would bring me to tears (I would cry when he wasn't around), but I was afraid of telling him for fear of crushing him. I knew I saw things differently from him, and I didn't think he would understand. Eventually, it got to where I couldn't hold back my tears, and he saw me crying often. He prodded me for answers, which I evaded. I told him it had nothing to do with him, that I was going through a phase. After several months, he sat me down and told me that I could tell him anything and that he just wanted me to be happy. I finally explained to him my quandary, that I loved him and I didn't want to hurt him, but I wasn't satisfied.
He surprised me by taking it very well, and giving me reign to explore sexual relationships with other people. We've had a sexually open relationship for a while now, and so far it's gone smoothly. I'm certainly not saying that will be the case for everyone, though.
At this point, it's my personal opinion that it's ridiculous to EXPECT one person to completely fulfill any aspect of someone's life. People usually have no qualms with their significant others having (non-sexual) friends, which help satisfy emotional needs...it's the view of so many people that it's fine for their loved ones to seek solace in the minds of others as long as they "own" their bodies. It seems strange what things people will take personally and what they won't.
All that aside, it may not be the healthiest of circumstances if she has a specific person in mind...though the fact that she told you is a very good sign.