How...?
It seems to me as a woman, if this woman is indeed a friend of yours then I would hope you two talk with each other. If you have trust and some emotional intimacy you might if this is not too risky for you, talk with her about tickling, or fantasies, or some related topic.
Then segue into a discussion about tickling. If you value this friend and the friendship -- be a gentle person and communicate. Do not just get advice on how to wildly tickle this woman. That is not what it is about.