Airport Operations Manager reported observing an aircraft take off and come within 50 feet of a landing aircraft that was still on the same runway.

Narrative

I am the Airport Operations Manager of ZZZ. Aircraft X was on short final; Aircraft Y was holding short of Runway XX; Aircraft Y radioed Aircraft X asking his distance and speed; Aircraft Y answered. Aircraft X landing midfield; slowed to a stop; turned around and called a taxi back to nearest runway exit. Aircraft Y; radioed out he was taking the active runway for departure. I radioed Aircraft Y telling him there is an aircraft on the runway. He did not respond. He then throttled up; and began his rollout. I radioed again; telling him to stop; there is an aircraft on the runway. He responded with; 'I know; he's not a factor'. I again told him to stop and he kept going. Aircraft X was communicating to Aircraft Y he was on the runway. Aircraft X was unable to get onto the grass and instead sped up to the nearest runway exit; and right as he was beginning his turn off; Aircraft Y lifted off the ground right in front of him and came within 50' of Aircraft X. The Aircraft Y pilot is UNFIT TO FLY.