We are all nervous when our bunnies are about to have procedures done, it is always scary. Still it’s the most important thing we can do for a girl rabbit in order to keep her healthy long term. (I’m sure you know this, though, but it’s a good thing to keep in mind: As scary as this is, it’s in her very best interest).
You should make sure you get a painkiller to give her for at least a week, it’s very important to relieve pain in a newly spayed rabbit, there is scientific evidence of that.
If your bunnies are bonded you shouldn’t separate them completely, they should absolutely be near each other, but direct physical contact may not be good the first 24 hours or so. That really depends on how they interact. Sometimes a bunny reacts to the strange scent of their bunny partner when it’s been to the vet, so it’s actually recommended that you take both buns with you to the vet – although the other bun will of course not be present during the surgical procedure. Bunnies heal way better in the presence of a friend.
You should feed her right up til you are going in, rabbits can’t vomit so they don’t have to fast before surgery, and their tummies should always have food in them in order to work properly. Get som Critical Care (or equivalent) and a syringe for feeding to keep on hand at home in case she refuses to eat when she comes home. And keep a variety of food that she likes on hand too – anything she wants to eat after her surgery is good, even if it’s fruit. It’s also important to keep her warm, bunnies that have been put under are prone to hypothermia. A hot water bottle wrapped in a blankie is often appreciated, but she must have enough space so she can move away from it if it gets too warm for her liking.
I wish you and her the best! I hope others will chime in here and offer additional advice!