-5G is no huge deal for a short time. That's 5G upwards, not sideways, but I don't really see that it would make a difference. The Pitts is rated for -5G which implies that people do it from time to time. -3G to -4G is fairly unpleasant but no more than that. In an outside loop you push a sustained -3G for 10-20 seconds. It gives me a headache especially after two or three, but that's all (which is why I don't do them any more).
A more likely explanation is that the sudden yaw whacked their heads against the cabin interior. Even then though... it wouldn't be any more violent than a tumble initiation (Lomcevak etc). I've had plenty of bruises from that but I don't think it would knock you unconscious.
Of course after whirling around like that for several minutes, it's no surprise there was blood. But in the first couple of seconds..?
All very odd.