My career is in the car business. I have driven the SC430, Z4 and 330CiC. Yes, the SC 430 and the 330 are closer comparisons (though I would say the Mercedes CLK is a closer comparison), but they are targeted at different markets and are designed for different purposes. The Z4 is a roadster/sports car, where the SC is a luxury convertible. If your friend is looking for a fun, noisy, throw the car into corners type car, then the obvious choice is a Z4. If your friend is looking for a quiet, plush, impress your neighbors car, then the SC is the obvious choice.
Keep in mind that the Z4 is made in South Carolina (yes, this does make a difference), the dealers throw away Z4's at the end of the model year (there are lots of leftovers - they are not very strong sellers), and the resale value is very weak - the car does not lease well due to the low residual. There is also quite a bit of dealer cash, which also weakens resale value.
If your friend is looking for a sporty roadster, a Porsche Boxster should be one to look at. I chose the Boxster over the Z4, Miata Speed and the 911. The only thing I sometimes question is: Should I have spent a little more and got the 911? Then reality hits and I realize an extra $600/month doesn't buy me that much more pleasure.