There may have been other circumstances that prevented him from calling someone (perhaps his phone wasn't on him)

I don't really want to get into who's fault it really is and all that because it really is circumstantial, but I do want to say it isn't the taxi driver's fault. You pick someone off and drive them to their location. If they don't request that he stay and make sure x happens, then he can simply go and find another fare. That's how taxis work. He was simply making a routine run. Just because the man died after he left the taxi doesn't mean it's the taxi's fault.