Check that the actual server is visible from the client computer, and that there are no firewalls in place that might block SQL Server:
Configure Windows Firewall - SQL Server | Microsoft Docs[
^]
It's pretty obvious, but check the server he is connecting to does have the DB installed under SQL - if you are Attaching a DB for example then that will only work to Express editions of SQL Server, not Production.
Then check what the actual server he is using has configured in terms of users and passwords: unless he is using the same server as you it's likely that the users are not set up.
Also look at any error messages generated by the connection process - they can often tell you what area the problem is.