Should I get upset about my friend telling me about her fling with her mentor?
Tell me more. Why would that make you upset?
She was supposed to tell me more about it and didn't.
I think you are being selfish in this situation. You shouldn't upset friends when they share secrets with you. I think you should be more understanding and happy with what they came to you about.
She was suppose to tell me more in detail but didn't.
Maybe the timing isn't appropriate and when she is ready, I am sure she will tell you more about it. You just have to be more patient and not pry too hard. It sounds like a sensitive topic, so it's not easy to just say everything without thinking about the consequences.