Once I learned he was connecting to a named instance, I had him check for the SQL Server Browser Service. That was one of those sample questions that was on just about every sample test for the MCITP exams, so it was drilled into my head...sure enough, it was disabled. If you're connecting to named instances (unless you provide the port number as well), you'll need the Browser service running to make sure your connection lands at the appropriate spot.
So a very simple tip today but one I was pleased to see make it out of the academic side and into practical usage, for me.
the browser service
is essential to connect
to named instances