No, not necessarily. If he uses the same laptop/PC, the drive letter would usually stay the same. If he uses multiple USB devices at the same time, it may differ. He'll want to always plug his USB drive in first. The System.Configuration assembly will need to be added as a reference if you do not already have it.
Here's how you get to it in C#:
<add name="myconnectionstring" connectionString="the connection string goes here" />
C# code to get value:
I got to be honest with you. Working with configuration files is pretty basic stuff. Does your client know how inexperienced you are developing WPF applications?