I am pretty sure that if he is working in MT he is required to get a MT DL and put MT plates on his vehicles. Six months is way beyond the limit for a working person to be without a local DL and plates in MT. Retired folks excepted.
The bright side is that in MT you can get inexpensive forever plates on your vehicles. Many faraway folk create a MT LLC just so that they can cheaply license their lambo for a one time fee without leaving their NY or LA crib. Your friend will also want a MT DL so that over the counter elk, bear, and wolf licenses can be bought without hocking a kidney.
The bright side is that in MT you can get inexpensive forever plates on your vehicles. Many faraway folk create a MT LLC just so that they can cheaply license their lambo for a one time fee without leaving their NY or LA crib. Your friend will also want a MT DL so that over the counter elk, bear, and wolf licenses can be bought without hocking a kidney.