Yes, white tie event (gentlemen in tails) so she really should have been in floor length. And she should know that stuff.
Cefinn not really suitable, though they do have a few dresses in opulent fabrics. They are mainly maxi-that-still-shows-your-feet rather than proper floor length, but would have done rather better than one that short. But most of their so-called "maxis" are really midi (in its proper sense of mid-calf, and with knee-length skirts wrongly described as midi, so a frustrating site to find what you want)
(Maybe she secretly hates all that stuff and is hoping to escape from most of it, and firmly intends to signal that at every opportunity!?)