Don't get involved in the billing or you will be out of pocket for Google clicks and chasing them for payment. Instead, set up the account in their name.
Next, create your own MCC account here: http://www.google.com/intl/en/adwords/myclientcenter/
Then you can login to their account and add your own account as administrator.
This way you can manage their account, but you are not liable for click payments. Also, if you part ways later, they get to keep their account.
