I know it is so stressful.
Please, whatever you do, only let her stay there overnight if someone is there with her AWAKE the ENTIRE time. I would not consider letting her stay overnight by herself. It is very dangerous to do so. If they leave at night, insist on taking her home.
There is just too much that can go wrong. Incisions could split (very rare), they could react to the pain med or they could not come out of anesthesia right and their life would be at risk. There also needs to be someone there in case she is in extreme pain and meds wear off.
Ellie goes to a vet office that is staffed 24 hours and they still let spays come home same day. Ellie came home about four and a half hours after a spay, liver biopsy and teeth cleaning.