He should rinse his mouth a few times a day with very warm salt water(as warm as he can comfortably tolerate. Use one teaspoon of salt in a cup of water). This will help reduce the pain and help heal any infection that might be there. Often people have some discomfort after a crown is installed. This should decrease with time.

Diarrhea and nausea could be due to an infection. Are you taking in too much salt? If your blood pressure is high, you should not be taking in licorice root. Perhaps you should take extra vitamin c, olive leaf extract, and raw garlic. Are you taking in enough potassium? Fruits, vegetables, and milk are all high in potassium. Do you drink enough water? Even just dehydration can cause nausea. Parasites can also cause the symptoms you have.