What Illness Or Disease Might My Friends Cat Have?

This is a female cat about 10 to 12 years old. For about a week, she won't eat or drink anything on her own. She's always been a healthy cat. Her rib cage and belly area has shrunk but her front and rear quarters appear to be normal. She's also been lethargic. She's an indoor/outdoor cat. She's being force fed through a syringe, a mixture of baby food, pedialyte and antibiotic (erythromyacin?) but she is only urinating but not having bowel movements. Please help me or she'll be put down by her owner.