So this scheme achieves consensus via the trusted method of asking a friend. It will probably work but it won't scale.
Interesting. Scalability. Could you elaborate more on why not?
So a fork occurs at some point and an exchange has to decide which fork it should take. Does the exchange owner have to contact all his customers to see which forks they are on, contact other exchanges to see what forks they are on. And what happens if some people disagree? That doesn't sound very scalable to me.