Animation and Rigging Tools: FAQ, Known Issues and Feature Roadmap

You’ll need to have them do the installation part (including copying the userSetup file to overwrite their own), where they’ll point maya to the MayaTools folder location.
https://docs.unrealengine.com/latest/INT/Engine/Content/Tools/MayaRiggingTool/index.html

Then they should be able to go to the Epic Games menu, and add character for animation, which will add the character and open the UI. You can also access the UI from the Epic Games menu as well. I guess the most important thing is, they need to have that menu for the UI to show up.
Hopefully that was the issue. Let me know if it wasn’t!