Even if they don't have an Enterprise account, they can add you as a user on their account. They need to login to developer.apple.com, then click on the People tab. They need to add you there. Here is some good docs on what I just described.
They will also need to add you to the iTunesConnect account. Unfortunately, for some reason Apple doesn't allow you to have the same email address linked to multiple accounts, so the client will need to add you iTunesConnect account, but make sure whichever email they add isn't linked to another account. This is another doc on what I just described
Once that is setup, you'll have access to the account, but you'll never see their credentials, and they can remove you whenever.