My friend's car seems to stall a bit more than half the time on cold starts. What he does is revs on the engine for around a minute until it warms up so it has less chance of stalling.

After it is warmed up, it doesnt have that problem.

What's the first thing to check on this? or what should be done bout this?

It's a 98' grand prix gtp
around 80,000 miles

Check that the car has a thermostat. If it does looks like it stuck open. The ECU(PCM) can not work well on an engine that is not at is working temp or reaching it in a small amount of time

A thermostat stuck open will do nothing more than throw a code.

Have you tried cleaning your IAC valve?

JoeJoe: If the IAC is the problem would be happening all the time not just when the car is cold. When the car never reach the operating temp or takes a while to get there you have to warm it before use it like what krz depicts here.

Krz, never the less. Check that the IAC opening on the TB is carbon free as the TB plate circunference and walls. I incline more for the tstat but never hurts to keep the intake tb plate and IAC opening clean

On my car, that problem was Bad Coolant Sensor. I've replaced that and the problem goes off.

Try it.