Father 90, has angina and dementia, could he have major surgery?

Ask his doctor. Assessing the risks and benefits of surgery depends on patient factors and the type of surgery. Consult with an internal medicine physician or the surgeon.
Surgery. At this age he would be at high risk for stroke, heart attack and even death with any major surgery.
It depends. This depends on what surgery is needed, and reviewing the risks and benefits to your father. His physicians (including cardiologists, primary care doctors, and surgeons, etc) are the best to evaluate these issues.
Look for alternative. Although there have been exceptions, because of the need to cooperate during recovery, one would attempt to select therapy which might provide a solution without surgery. Often one could open one or more coronaries with stenting.