I don't think she does.
If you are 300km (somewhere around the humber bridge?) from the end with 10 hours remaining, you could theoretically make it if you can keep up 30kph average including any stopped time, so a moving average above that. Very few people could actually do it, especially not the majority mere mortals and especially not after having already done 1200km, but maybe it could be applicable in one of those edge cases e.g. where a super-speedy person has had to have a long stop to buy a new frame (or weld their frame back together again) and then rebuild their bike before attempting a heroic flat out dash to the end despite being out of time at some intermediate controls, in a way that tells a good story, and might feel unfair to DQ.
At 12kph you'd have 25 hours left to complete the ride, and you'd be within the normal control times. There's no need to surrender a brevet card, you're on track, and have over a day left to get to Debden. Get pedalling!
If you were at the same point with just 9 hours left to do those 300km, requiring an average speed in excess of 30kph, it wouldn't feel unfair to remove the brevet card and say 'your ride is over' because you couldn't do it while respecting, over the remaining controls, the maximum average speed allowed in the rules even if you were that super rare bird that was capable of it.