Bring the chicken stock, salt, and butter to a boil in a medium saucepan. Stir in the couscous, cover tightly with a lid, then remove from the heat. Let the couscous steam for 5 minutes. Fluff the couscous with a fork to break up any clumps.
While the couscous steams, heat the oil in a medium skillet over medium heat.
Add the peppers to the skillet and cook until tender, about 4 minutes. Stir in the garlic and chicken then cook until the chicken is heated through, 3 to 4 additional minutes. Season to taste with salt and pepper.
Fold the bell pepper and chicken mixture into the couscous to serve.