The goal of his attackers is to eviscerate him to harvest his organs, whatever can be accessed through the abdominal wall. The victim is not supposed to survive, though - as he is one of the protagonists - he better has to.
The attack is stopped by his friends in the very first stage, the abdomen has been opened, the inner organs are accessible, the incision is roughly 10 inches long, nothing has spilled out, but sepsis/peritonitis is definitely in the offing, as the peritoneal cavity has been compromised. All the innards are however still where they belong.
The basic situation is: outdoors, cold, stormy weather, unreachable by any kind of rescue craft, everyone is on horseback, there's a first aid kit including suture material (catgut and coarse staples, both actually geared to repair horse injuries) available, which also contains antibiotics commonly used on horses, something like Penicillin or Amoxicillin or some vet usage broad band antibiotic. Also available is a person able to suture competently deep muscle wounds and skin (on animals). No anesthetics are available.
The guy has to be repaired enough so he can be transported by travois or a similar device for at least a day, if not two days and survive the whole thing without any later defects.
I searched for peritonitis, evisceration, opened abdomen, amoxicillin +human usage, penicillin +human usage. Results were mixed and did not answer my most pressing questions:
1. How much of a blood loss will he have? No major blood vessels are cut, only the entire abdominal wall clean through.
2. Will someone able to suture e.g. a similar wound or a major muscle injury on a horse be also able to do a competent job of sewing those 10 inches back together on a human without causing more harm than good? Person is cold-blooded and surehanded.
3. If the wound was kept relatively clean (no dirt fell into it, no one pawed inside it, the knife was not sterile, but razor sharp and clean as in dishwasher washed clean) and the injured person received a goodsized injection of antibiotics, or maybe two of those spaced 24 hours apart, would he suffer from peritonitis or sepsis? Or would he be alright?
4. Which of the common broadband antibiotics used on horses can be used for such a situation and a human and in what dosage to keep him from having a sepsis?
5. Where - if the goal is harvesting organs - would the incision be situated, if it is the first cut towards the described purpose?