1991 626 does not idle in neatrual

My friend has a 1991 mazda 626 that starts up ok. If we puts the car in first gear the car will stay running, but every time he shifts from 1st to 2nd gear it will died out. It seems that the car will keep on running as long as it is in gear. The minute he shifts into neutral the car dies. Any ideas, let me know. THanks in advance.

noel

My friend also forgot to mention that someone replace the timing belt on the car before he had these problems. If the timing is off, would this cause the car not to run properly or die...