Good to know.  If there's another entity between your client as an individual and the US LLC, the outcome could be different.

In that case, your client will likely need to file the return on paper but make sure the proper process is followed going forward.

Note, however, states generally do not recognize income tax treaties although a few do have clearly stated positions.  You should review whether your client's income would be subject to state income tax if you have not already done so.

Hope this helps.