Fractional First Officer reported while on short final observing an aircraft enter the runway midfield facing the opposite direction resulting in the fractional aircraft executing a go around to avoid the conflict.

Narrative

We were on short final landing Runway XX at ZZZ; approximately 300ft AGL when another airplane; Aircraft Y (we found later from the FBO staff) entered the runway at the midfield taxiway (approximately 3000 feet down) and turned to begin a take off roll from Runway XY. A go around was immediately started by the pilot in command (PIC) (pilot flying (PF)). The plane was cleaned up; and the go around was called. We entered the left downwind for Runway XX and returned to land without further incident. The departing aircraft took off and left the area; with the only radio call made being sorry about that".After shutdown; the FBO staff stated that they heard all of our radio calls; but the departing traffic had made none. They stated he was a transient and didn't know where he was from or where he was going.Our TCAS was inop (MEL) so no traffic information was received.While cumbersome and sometimes probably impossible; minimizing the use of uncontrolled airports by aircraft with inop TCAS would be a huge benefit to safety."