I'm not a doctor, only a former afib patient. But as I understand it, Afib is an "episodic" disease. You are either in afib or you're not. When I was in between episodes, I had zero issues, heart was 100% fine and that's pretty common.

Some reasons for a heavy-lifting restriction could be A) he has other heart/cardiovascular issues besides just the Afib or B) they are concerned heavy lifting could trigger an afib episode.

Everyone has different triggers, most commonly alcohol, stress, and endurance cardio training. I've not heard of heavy-lifting being a trigger but I suppose it could be.

Is he on a blood-thinner? Commonly when doctors are concerned about afib they will put the patient on one as a precaution against stroke, since you can't instantly teleport to a hospital when an episode begins. Stroke is by far the biggest danger of afib - the other symptoms don't typically cause major issues aside from fatigue, discomfort, etc.