My friend wants to do just heads on his stock '98 Z28. What will this do for him?

 

it will give him more horsepower and torque. depending on the head, 20-30 RWHP, and 15-22 Tq. You should advise him that he may as well do headers at the same time because:
a) he's taking the manifolds off anyway
b) it makes the header install 300% easier
c) he'll see another 20HP and 20-25TQ with headers in addition to heads

 

Not with a stock cam he won't. Need a cam to take full advantage.

 

I would recommend doing a cam swap instead of heads. Heads are extremely exspensive so doing a cam would not only save you money but you'll get more power. Don't forget headers will be needed to take full effect of a cam.

 

Yea if you look at people who do a cam swap and then follow up with a head swap, you usually see the cam attributing for about 70% of the power increase. And the heads are really putting that 30% extra on merely because the cam holds the valves open long enough for them to flow some serious air.