Normally, if the ticket doesn’t say anything to the contrary, you can cancel up to the scheduled departure time. The closest I ever cancelled a ticker (not CX though) was approx 1h pre departure.

I’m not sure how close-in you can actually cancel, i.e. if the flight is already closed but is still pre departure time, can you still cancel or are you deemed a no-show at this point?

12h should definitely be fine but if your friend is already checked in, she will usually have to cancel the checkin first.