My husband has atrial fibrillation and today while walking had a 5-minute episode of burning left-sided chest discomfort. Should we be concerned?

Yes. If it occurs again tonight, you should call the answering service for the cardiologist or go to the er. Chest pain with physical activity is always a concern, especially in someone who already has heart disease like a fib. Regardless one of your husbands doctors should be made aware of this in the morning if no symptoms recur sooner.
Sign of coronary dis. I would be concerned. We must rule out a coronary artery disease like angina or a heart attack . I hope that you already consulted with his cardiologist or taken him to the er by the time you read my answer.