If you're in California where lane splitting is legal, I'd say that as far as legality goes, the cage is at fault here. If not, it seems like 50-50 fault because both drivers were breaking the law.
Judging by the description, and as far as responsibility goes, i'd say the squid should have probably slowed down a little bit if he can't handle an emergency stop at relatively low speeds. But let's be honest here, lane splitting is dangerous and there are situations that even the most experienced rider would not be able to avoid. If you are in a car's blind spot, and he cuts his wheel as hard as he can to make an impromptu lane change without checking, you could definitely become a pancake regardless of your skills or reaction time.
By virtue of lane splitting alone the rider is putting himself in a precarious position and is definitely responsible if something happens. The rider should know that most people don't check or care if a motorcycle is lane splitting, or even not lane splitting for that matter. Our safety is definitely our own responsibility, and we understand that in some case safety can be out of our control, most especially during street riding. I hope the guy is OK and wish you would have gotten it on GoPro.