A friend of mine has a preban Colt AR15 will postban uppers fit on this rifle? Other than the fact that it's illegal can you put a preban upper on a post ban lower??

All uppers and lowers are interchangeable. You just cannot legally put a pre-ban upper on a post-ban lower, but you already knew that.

Thanks M4... That's what I thought but not having one myself (yet) I didn't want to steer him wrong...

91101, while pre-ban uppers and post-ban uppers are mostly interchangeable, don't forget about Colt's screwing around with the milspec pivot pin and holes.  At one point, Colt began replacing the small (standard) hole on their lowers with a larger hole, so that M16 and other "bad" uppers could not be used.  An adapter was developed to get around this.  Check your holes in your lower to see if they are large or small (I don't know the exact sizes).  You might need to get the adapter to mate the two.