hey. i wonder if it would help to talk about it more abstractly... then you could gage her response and assess whether it would be safe for you to tell her more.

have you seen a movie where something similar has happened? could you talk to her about that?

could you perhaps introduce the issue in the sense of 'you have a friend who has a friend and they have become a little more than friendly and one of their mothers found out and gave your friend a bit of a hard time...' and then see how she responds to that... and if she responds well then it might be easier for you to tell her that you are your friend?