My friends key starts my car
 

Just got a 88 turbo ii and My friend I work with has a 91 turbo ii swap, he got in my car and was able to start my car using his rx7 key and drove around the block while we were working, is there anyway to prevent him from starting my car ?

why not steal HIS car?..that is the question!

and you can get another ignition and lock setup (used) if you really,I mean REALLY feel that he is gonna steal your car.
But I'd just get drunk with him and bitch about all the money that you spent on it and how many problems you have had with it and BOOOM..instant Theft Deterrent System!

Doesn't surprise me, back in the day when GM had the seperate door and ignition keys, rul of thumb was there was 2 alikes every 10 keys

There wasn't much of a key spread on the RX-7s. And at this point, all the locks are worn out anyway.

If care, you can re-key it. But the re-key kits are fairly expensive. I had intended to do my house locks but faced with the price of pin/re-key kit it is cheaper to buy new cylinders.

Worn keys and cylinders were my first thought too.
Option 2...maybe get a quality switch and wire in to the ignition at an unobtrusive spot. That's what I did on an old 70's era Chevy truck I had. Same issue, worn keys, worn ignition and too few combinations. That and you could break into and steal 'em with a butter knife and screwdriver.