Ended up figuring out from another forum post: How do I call a method on another verse device?
Basically for my example @editable then reference the other script, and then it can be called.
game_manager:= class(creative_device):
@editable
hello_world_device_ref: hello_world_device= hello_world_device{}
OnBegin<override>()<suspends>:void=
hello_world_device_ref.Foo()