I completed this gig awhile ago. I eliminated the guy and the quest didnt finish. I then got frustrated and tried to blow his head off. Wouldnt happen so then I tried to throw em into the river. I got him in my trunk, drove to the rive and couldnt throw his body because that mechanic was buggy. So I gave up left his body in the middle of the bridge and drove away and did something else. His body has now despawned. Along with the quest marker being gone.