Well, that's part of the problem. It's a dedicated server in a local hosting company (Arsalon). I only have access to the machine over SSH, and Redhat Enterprise Linux (I believe) is not a free download. I did insist that they take a complete back up of the system before we start, so that will exist.
I am not a lawyer and I don't know what parts of RHEL are proprietary but I would be very surprised to hear that by restoring the backup of the live system onto the test box and testing on it you would be violating anything, and even more suprised if in the surprising case that you were violating anything, that anything negative would come of it. The client has a license to the proprietary parts, and you probably aren't going to be touching the proprietary parts anyway in the test box. You would be operating in good faith within the terms of your client's licence to RHEL.