By default, BIDS doesn't overwrite datasources on the report manager website so unless your user has created new datasources and deployed them then I suspect they might not actually have deployed datasources before. Easy to test by creating the datasource with a new name in BIDS and testing both the connection and the deployment (does the new datasource appear in report manager).

But in your question you say the user has problems "creating" the datasource in report builder yet they've had no problems "deploying" from BIDS, which might be a second issue.

Creating a datasource in report builder involves both permission to connect to the datasource using whatever credentials are supplied (does this part work? does it work for that exact same datasource in BIDS?) as well as saving the object in the datasource folder on the server. If the user (on their same machine with the same windows login using BIDS) is able to deploy a new datasource to the server, then there is no reason that report builder would deny them access - both processes use the current windows login for permissions.

I would go back to the user and double check those things.