Things to check:
If the user uses roaming profiles and/or has his VS add-in directory on a network drive / UNC path... ...the assemblies will not be trusted, and VS will fail to load the add-in.
Also make sure that they have the AddIn checked to load and run...
On the Tools menu, click Add-In Manager.
Is the Add-in registered to run with Visual Studio ?
Can you check that the Add-In dll is installed in the path you asked it to ?
(for locations: See File Locations)