Do you not have any access to the registrar account where the .com is held? Alternatively if the domain is in the client's name then you'll need to go to the icann site (network solutions?) and complete a form and fax it to them in the US to get the details changed. I've done this in the past and it was dealt with within a few days. If you can access the domain settings then you'll need to get an authorisation code from the current registrar and get the domain unlocked. Then once you request the transfer the admin contact shouldn't need contacting as an unlocked domain should transfer without interference.
Mind you, if you can gain access to the admin for the domain you can change the admin, billing and technical contacts yourself. Well you can in eNom
