Tranny Problem

I work with a guy with a 92 colt. 1.5L 4sp. He said it started slipping out of 4th about three months ago, so he started putting a bungee cord on it to hold it in gear. Now he can't even keep it in gear by hand. His mechanic said it was probably some sending unit and te tranny would need rebuilt, but it sounds to me like the cable or linkage is out of adjustment. I don't know much about trannys or colts so any advise would be appreciated.
Dustin

:meant: His Mechanic said it was a syncro going bad, witch is at least part of a manual tranny, but I still don't think it would keep it from staying in gear once he got it in.

yes it would not only do syncros let it go into gear it also keeps it from coming out of gear, google manual transmission in pictures and see if you can get a look at the inside of one and see how it works if its not on google try to find a websit that is called how stuff works and it should show you its hard to explain how one works its just easier to show someone.