I also thing that any tolerable exercise is better than no exercise. Getting out and about gives one's sense of autonomy and health-problems a break from anxiety and continuous reliance on one's sensations or wearable devices for continuous feedback.
Unfortunately, as it is a progressive disorder, AF will tend to evolve out of the previously prescribed regimen, whether it be weight loss, diet changes for other reasons, avoidance of unique and common 'triggers' (caffeine and alcohol are the two biggest most agree can cause a run of PAC's or AF), anti-coagulants, and the many anti-arrhythmic medications. His cardiologist or elecro-physiologist might place him on a stronger dose of what he's taking, or change the drug to another. That's the trend for many of us....more power, as Ted the Tool Man would say.