I've had many, many females spayed (some mine some fosters) and never has any one of them had to stay overnight.
If there was a medical problem during the spay, then it would be best to have them stay, but only if someone will be with them overnight.
Even if the Vet. was an hour or two drive one way, I'd stay in a motel just so I could take her out on the same day of spaying and/or a return check the next day.
Have you ask why they would even want to keep her overnight and especially without supervision? |