The problem with antique registration in Montana (and this wasn't made clear on the state website) is that you can only drive an antique to and from parades and/or car shows. I had originally planned to register it with antique plates because it's more affordable than a standard registration. That was a deal breaker though.
https://doj.mt.gov/driving/license-plates/#antiqueplates
Although by driving it to work he probably violates the 'general transportation' provision.