BOTTLEDZ28

my buddy is having a problem with his 1996 z28. Ever since he bought his car, he has had to unlock the drivers door with the key and hurry up and get in the car and put the ign key into the column and turn the key to the run position before the horn would start beeping. It has worked fine up until he started having problems with starting his car. I figured it was the key worn out and I ended up bypassing the PASS KEY using a resisiter. This worked for a while but now he just got his car back from the shop where they rebuilt his steering column and now the shop owner was telling him that the lock cylinder on his drivers door is worn out and it is not turning all the way to "disable the alarm". Can someone explain to me how this sensor inside the door works with the alarm system and can it be bypassed? The shop owner told him he has to buy a new cylinder and sensor unit from the dealer or he will not be able to start his car. Someone please explain this.