Not a silly question - but I did try that as well. The same problem. What's odd is that it's not a 'server not found' error (or in fact a redirect to their main site, as they have with sub-domains that definitely do not exist), but rather a 500 Internal Error on their host.
Good. Then the problem is between them and their host. Unless you've got formal authorization to act on their behalf, you couldn't do anything if you wanted to* ;) If it were a DNS issue you'd never even get that far.
But you need to look at the error logs for more information. Even at the generic shared-hosting level, error logs will generally tell you enough to diagnose the problem.
* Assuming for the sake of discussion it isn't one of those "What's a browser?" clients. If so, it will be faster to get authorization and do it yourself than to explain what to do.