If you well and truly value your friendship, I would recommend you cut off sexual contact with her.
Two and a half years ago, I lost my best friend, who was curious, knew my orientation and had a boyfriend she wanted to please. There were a couple of encounters between the two and three of us and while some of it was good, I fell hard for them, she turned out not to like women, and became frightened by the connection developing between me and the boyfriend.
I still miss her.
That said, I could see all the warning signals and still plunged ahead. We all have to make our own mistakes.