That's awful smow :( There's a worrying amount of poor views on Trustpilot, filtered here to the worst two categories, and some with similar issues. Though many who have had deliveries aren't happy either.
Concerning how recent so many of them are too, page 6 of the poor reviews still only goes back 2 days!
https://uk.trustpilot.com/review/www.eflorist.co.uk?stars=1&stars=2
It sounds like they use Yodel for delivery, they are mentioned in quite a few reviews, and given the description of the flowers some have got I'd be surprised if they use decent local florists, I'm wondering if they come from somewhere more central, hence the issues people are describing.
A few things cropped up a few times in the reviews. As they seem to use Yodel, could there be somewhere the flowers could have been left that your MIL might not have seen them?
There also seems to be some kind of disclaimer about how deliveries can take longer due to Covid, though that suggests maybe days. Some people say they've had emails about them not being able to fulfill the order, so might be worth checking your spam folder, just in case something came through.
I really hope you get it sorted.