Give your girl a good, solid breakfast before you take her to the vets, as you don't know how soon he'll operate. Grazers need regular food, and piggies can't vomit (which is the reason for fasting before an op).
Make sure that Lily has plenty of rest for the first two or three days. Don't worry, if she doesn't eat lots, as long as she nibbles a bit, especially on the first day - her whole inside is pretty churned up and it takes time to settle! She should get perkier with every day, but sleep a lot at first. Give her cucumber to nibble, so she had fluids until she feels like getting up for a drink.
Please make sure that you have probiotic, critical care and painkiller (metacam or rimadyl; or calpol from the pharmacy, if the vet refuses to) to make her as comfy as possible and get her bowels going as quickly as possible. She won't poo much or have funny poos at first, until she eats fully again.
See how Darcy gets on with her when she perks up - he may be too much for her at first. If you can keep them close for company that would be good. Be aware that Lily will smell strange after the op, and Darcy might react funnily or go overboard. O the other hand - he can be very sweet and cuddly; you have to see!
My guinea pigs loved a soft, covered cuddle cup to sleep and recover in after the op.
Wishing her all the best and a good, speedy recovery.
We will be here to "hold" your hand, if you wish! It helped me a lot while I was worried sick about one of mine at the vet's!