You don't necessarily have to get pc2100. pc2100, pc2700, and pc3200 should all work with that motherboard. If you put pc3200 in the motherboard it will downclock to the speed that the cpu is running at. pc3200 is probably cheaper to buy then pc2100 right now, so might as well go for the better cheaper stick
Some more info about RAM. Don't worry too much about timings. PC3200 that has a CAS latency of 3.0 will easily have a CAS Latency of 2.0 if downclocked to pc2100 speeds. The only time where identical sticks with exact timings are needed is when you are running dual channel.
The last part of info is the following question. Why does he need 3 GB of RAM? I don't think your friend will see any difference between 1 GB and 3 GB of RAM.