Ground Controller reported a ground conflict when C-172 passed hold lines and was near runway edge when DHC-8 landed.

Narrative

Due in part that GA Runway has been closed due to construction it has been necessary to mix slower traffic with faster traffic on one of the two remaining runway's. This was the case where Aircraft X landed Runway XXL; at night; and became disorientated as to where to exit the runway. Aircraft X was followed by a DHC-8 two miles in-trail and I heard the Local Controller attempt to expedite Aircraft X to turn right at Foxtrot to ensure runway separation with Aircraft Y. The Local Controller then advised Aircraft X to make a 180 and hold short of Runway 30L. I observed Aircraft X exit at Foxtrot and made a quick 180 and pulled right up to the edge of the runway disregarding hold line markings. I advised the Local Controller it appeared Aircraft X was not clear as he was right up to the edge of the runway and the Local Controller responded; 'screw it; he said he was clear.' I estimate that Aircraft Y passed Aircraft X within 20-30 FT. Due to lighting conditions I do not believe Aircraft Y ever saw Aircraft X off his right. We are currently installing LCGS (Low Cost Ground Surveillance) which may have indicated the location of Aircraft X and allowed the Controller to make a better decision based on proximity. In addition I believe the Controller should have been more specific and advised Aircraft X to go past the hold bars and then make a 180 and ensure he was clear of the runway. The Controller panicked and you could hear it in his voice.