If it is a mekach ta’us the marriage itself is invalid, so how can you say there is a possibility the kesuba would be valid?
Because, if when he finds out, he has no complaints, he may agree to accept the higher kesuba payment anyway. On those grounds, I would think that it’s possible that the kesuba is valid. OTOH, I recognize that it’s entirely possible that they may need a new kesuba anyway.
The Wolf