Windows 7 enterprise

I don't normally ask these sort of questions in these forums as an IT pro, as I know people offer advice for free, but I don't know what else to do.

I am a self-employed computer engineer and a client has asked me if I can decrypt his hard drive. To do that, I have to disable BitLocker. (He used to use it in his last company, which he doesn't work for anymore). He gave me the User Name and Password to get in (it is on a domain). All went well, until I tried to turn BitLocker off. Even though I have put his details in again so I can turn it off as an administrator, it either says 'domain is not available' or 'this requires elevated administrator privileges'. Can I leave the domain without being part of it? (if you see what I mean)