Open carry of a loaded handgun is certainly legal in Colorado, including one in the tube! It doesn't matter whether your friend has a permit or not, or that he's out of state or not. Colorado is an open carry state, regardless. The sole exceptions being those you likely know about: schools, Denver city and county, and where posted, with permanent metal detectors and security types to wand you down.
As for state parks, I don't believe Denver has any authority to do anything apart from what the state requires. Hence, if a state park is outside the boundaries of the city and county of Denver, the state pre-emption law would apply. The Meyers decision exempted Denver due to the urban nature of Denver. A state park doesn't fit under that definition, so state pre-emption would still hold sway over their land, though it may be managed by employees of Denver.
I am not able at present to provide sources, but you can easily check with the Colorado State Parks website for folks to contact within the state park system who should know the score on your question.