you said he 'replaced it' meaning he replaced the whole distributor, or just the cap/rotor? the vortecs can knife-edge the distributor gear causing slop, which leads to misfires, espeically the p0300s. if he replaced the whole distributor than its VERY likely that he did not set it correctly because as was stated earlier, you have to have a proper scanner that reads the cam offset retard in real time, trying to adjust it with a timing light will not do anything. many people are unaware of that, not to mention that the dist. position is VERY sensitive, and even tightening down the dist. hold down can move it enough to put it out of spec.
not to mention that the damn vortec distributor is made of plastic and the hold down screws are known to strip or crack on the distributor body causing poor seating of the distributor cap. I had a situation where that caused a no-start issue with my truck when I had a distributor.
other than that we need to know what fuel pressure he's getting with key on engine off, as well as idling at least. FP during a short drive would be helpful too. he should also watch the fuel pressure after turning the key off and see how long it takes the system pressure to fall. we also need to know if the problem changes when the engine warms up or not.
type of plug info would be good too.
it could also be EGR related because when they get gunked up the valve can sometimes stick open causing issues, are the problems constantly happening, or is it only at idle? if it gets MUCH better above ~2k rpm, then the EGR may be an issue, but if the problems remain at higher RPM, then its less likely the EGR is involved.
has he checked cylinder leakdown or pressure by any chance?
has he done any modifications to the motor?
unfortunately a P0300 is sometimes VERY hard to track down, I've seen it caused by any of the things mentioned above, I've even seen a bad ground wiring problem cause it.
