You have to make her truly understand that you love her for reasons other than sex, and that you want to see her happy and having a sex life that maximizes her pleasure.
She's likely afraid she will lose your friendship and support if she were to act on the desires of her inner-woman. You should be open about the areas in which you feel inadequate to properly please her, and encourage her to find a guy that can.
Make sure she understands that you're not encouraging her so that you can also find another woman, but that you feel inadequate to properly please any woman and you derive pleasure from seeing her well-fulfilled and happy.