From a non using perspective I can understand how hard it is to understand. Maybe he wasn’t clean? Or atleast fully! Cravings can come and go anytime, especially if under some sort of pressure/stress. Triggers are everywhere! A mood swing can be a trigger, even LIFE can be a trigger!
It’s how one manages their own responses to life unfortunately, and so if someone has unhealthy coping strategies when the chips are down then I’m afraid they fall back to that (as a crutch). To kick an addiction takes enormous strength and willpower, along with a s**t load of “tools” in the tool box.
It’s a journey and one journey that’s walked alone, well in terms of wanting help etc