We are going to Harbin China Jan 3-8. My husband has COPD. What should we do?

According to the weather forecast, the temperature will be -20C to -10C. It will be extreme cold. You should bring enough warm clothes. Besides, prepare common medicines in advance. Prevent colds and enhance the body immunity. Appropriate exercise, strengthen nutrition, flu vaccination in late autumn, it is advised to eat some immunity enhancing drugs. Hope helpful!

Should he wear a face mask?

Yes, he'd better wear a face mask in order to avoid the cold air. But the air quality will be good during that period of time. Don't worry.