Well its a great start that you and your husband can be so honest and accepting of eachother and allow eachother the freedom to pursue other people.
I am curious though, why do you say that being fully poly for her wouldn't be an option because she is a lesbian? I don't quite understand that line of thought.
I understand your friend is afraid of breaking social norms, and that certain comes with the territory when you are involved in love or sex outside of your marriage. Have you talked with her about exactly what she fears might happen if you pursue a romantic relationship? A lot of people seem to be afraid of something different and out of the norm, but when you confront those fears and find the route, you can find ways to deal with it or conquer it. Is she afraid of people finding out, or of how it would impact her dating life with other people? If you know more about what exactly she is worried about, that might help getting around those fears.
I think you should talk to her more. Find out what she would want from a relationship with you if fear were not a factor. Find out what her fears are and if they can be overcome. Figure out if she actually has the desire to have a romantic relationship with you, and if fear is standing in the way, or if she just doesn't want that right now. Once you have some more answers it might be easier to work things out.