How do they know it is severe with only a blood test? Blood work cannot diagnose a liver shunt and cannot tell the severity:
"A liver shunt cannot be definitively diagnosed by blood work; shunting can only be found with advanced techniques such as scintigraphy, ultrasound, portography, Cat scan ("CT"), MRI, or exploratory surgery" Dr. Karen Tobias
Please see Dr. Tobias at UT. She is an expert on liver shunt in yorkies. 85% of the dogs she does surgery on for liver shunt survive and do very well long term. I'm sure something can be done. At this point, your vet is telling you to let her suffer when he doesn't even know what the problem really is. Sadly many vets are not very knowledgeable about liver shunt and many suggest putting down a dog that can live a normal life with surgery and/or diet.
http://www.vet.utk.edu/clinical/sacs/shunt/faq.shtml