Boiled mince or chicken and rice. boil the mince or chicken in one pot and strain to remove all fat. Cook rice seperatly and drain. mix about 50/50, slowly to normal diet.
An upset stomach can be caused by something as simple as grass. Make sure he or she gets water and bland food--plain rice and hamburger or chicken should be safe.
Give them something mild, like cooked white rice and chicken broth. If they are able to keep that down you can start slowly adding their regular food back.