Whether George is being an agent of chaos, staying in-character, or both, is a matter of interpretation.
If I'd had the room and inclination, Nanase could have elaborated further.
One reason such a random opportunity would require so much luck is that the bad guys would have had to stumble upon the knowledge that a guard was sick, and take advantage of that information without anyone else knowing he was sick.
If they found this out by keeping an eye on whoever made the guard's schedule, then someone else would have naturally been expected to take his place, and it wouldn't have presented the proposed opportunity.
So not impossible, but perhaps less probable than "a magic cleric did it" in a world that actually has magic clerics.
(It's also probably not the best plan just in general, but that's not on me. That's on the fictional writers of this introductory adventure. I'm in the clear.)