I recently treated a hen with peritonitis. First I kept her on a broad spectrum antibiotic which went in her water. After she went back to the coop she was fine for a few days then seemed to be backtracking so I got clavamox from my vet and gave it twice daily for another couple weeks. This seemed to keep any infection under wraps but wasn't a longterm fix and was doing a number on her digestive system. My vet gave me 3 options: 1) continue antibiotics 2) surgery to remove diseased plumbing followed by a couple weeks of restricting sunlight to get her to stop laying or 3) a hormone implant to stop her laying. I eventually went with the hormone implant. I didn't want to risk surgery because I know many birds die under anesthesia. The implant was expensive and I honestly felt a little crazy doing it, but what can I say, she's one of my favorites. I hope you have good luck with your hen. I think antibiotics are a good first course and hopefully will give the hen time to recover on her own or at least buy you time to decide what you want to do next.