How often did he change the water and when was the last time he changed it?
If the cloudy water is caused by the lack of water changes then nitrates could be quite high and pH quite low. A large change of water could kill the remaining fish. There are threads on TFF discussing this, search "Old Tank Syndrome".
If this is the case, and if the remaining fish swim and eat normally, then a small change of water, 10% to 15%, should be done daily until pH is slowly back to normal and nitrates gradually drop. This may take several weeks or a month.