I tried connecting to the site using a proxy server in China, and indeed I failed to reach Frese.EU, while I was able to reach dr.dk › forside (the Danish public broadcasting service).

Since there seems to be nothing on the site that could offend Chinese political sensibilities, it seems likely that this is the result of blocking of an IP address range. The site is hosted in a "web hotel" in the UK, and who knows who it may be sharing an IP address with.

Some options for your client if this is the problem:
1) Bring the hosting in-house, so it is their own IP address.
Put a linux box in their office.
2) Move to a different hosting provider and hope the new
IP address is better
3) Pay a few bucks extra for a dedicated IP address.

No 3 is what I would do. My company does exactly that, and we have never had a China problem.