If it's a static site you could have the files on both hosting providers, keep them up to date, then point the dns to one, but if it ever went down, dns could be changed to point to the other.
also, I beleive there might be a way - called round robin I think? That you can have it automatically fail over to the other host if te first becomes unavailable.
but if it's a database driven site then this wouldn't work.
i think you can also get high availability SQL clustering systems so if he wanted to spend a lot of money I'm sure you could build a failover / load balancing system based on those technologies.
lastly, might be worth introducing him to cloudflare - I'm sure that their enterprise packages offer 100% uptime guarantee through means of cached delivery on failure.