If they aren't on the same domain there is a lot of funny business you need to wade though. Otherwise you install the Visual Studio remote debugger on the server hosting the app and run it in admin mode and give the specific user permissions to debug.
Then they can go to debug-attach to process locally on their box and type in the box name and pull up the processes on that box and select it.
The PDB file must be deployed in the /server/bin/assembly directory for this to work.
Jamie Miley
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