Actually, there is a legal precedent for a passenger assuming command. It's called a hijack. That is exactly what this would have been classed as if the decision had been made to assume command, and rightly so - a B1-B pilot would know next to nothing about flying the 737, just as a 737 pilot would know nothing about the B1. It's one thing to offer help and advice, but is quite another to take control. Now, had the FO been incapacitated too, then that's a different matter.