You need to speak to whoever hosts the website. The showman will be paying them to host it, so it should be easy enough to find out who the site host is.
Explain to the website host that Dave Belton has passed, and ask them if they can reset the username and password so the Showman can access and administer it himself.
Once he has the details, he can give them to whoever he wants to administer it for him.
You can also search on a domain name to see who hosts it.