T'Pau could have simply assumed that Spock would have mentioned this detail to Kirk and McCoy (considering that they're supposed to be his best friends).
That would be a reasonable assumption in the absence of any other evidence. But when T'Pring challenges, McCoy says, "Ma'am, I don't understand. Are you trying to say that she rejected him? That she doesn't want him?" So it should be clear to T'Pau from McCoy's question that Kirk and McCoy have never even HEARD OF the challenge, much less know anything about what the subsequent fight will be like.
Given that challenging turns the woman into PROPERTY, I can well imagine that challenging doesn't happen too often.
T'Pau could have looked at the issue this way: Spock chose Kirk and McCoy to accompany him; therefore, it was Spock's responsibility to make sure they knew what the ceremony was about. From Kirk and McCoy's pov, since Spock never mentioned the Challenge, it should be reasonable to assume that he hadn't expected it to happen, and therefore didn't bother to mention it.
No matter which way you look at it, T'Pau obviously thought it wasn't her
fault that Spock hadn't told his friends that a fight to the death was a possibility.