Dilemma

I have a dilemma. Someone has told me that they are planning something as a
surprise for a certain persons birthday and sworn me to secrecy. The trouble
is that I am 99% certain that the recipient won't be impressed with this
particular something for a number of reasons, she has experienced it before
and vowed not to do it again. I have suggested that culprit re-thinks her
plans in the nicest possible way but I don't think I was very successful. So
now I don't know whether to tell the other party or not. Or perhaps I'm
being selfish as the event is likely to involve me as well and I'm 100%
certain I don't want to do it.