We are using OAuth credentials to access the Web Services API from a remote application. In general, this is working well. Our user authorizes our application using OAuth, and we are able to make API calls using their credentials.

However, one of our customers is having a problem. He authenticates using OAuth, but when we try to login to the API using his credentials, we received the following error: 

LOGIN_OAUTH_API_DISABLED: OAuth Api Access Disabled

From the documentation: Either the Force.com Web services API is not enabled for the organization, or OAuth API access has been disabled for the organization.

Our customer claims to be on Enterprise Edition, so he should have API access. How can our customer enable OAuth API access?

Thanks,

Shannon Hardt