An uneven number would look best in such small numbers, so either 3 or 5 male guppies would look nice in a 5 gallon tank. There are people who have a lot more guppies per gallon in their tanks, but they supply good filtration and are keeping the water well, doing regular water changes.
Your friend being a beginner would probably be more successful trying it first with three guppies!
Sounds good to me. With "after a while" you mean a few months, right? A small tank such as this needs a lot of maintance to remain balanced. If he manages to keep the three guppies alive he may get bit by the MTS bug (Multible Tank Syndrome lol) and decide to get a bigger tank. They are a lot easier to maintain and he'll have a lot more options in stocking it!!!
I believe those small tanks are just a lure anyways (I know I took the bait lol)! It doesn't take one long to realize what a waste they are! The only plus side is, you can always use them as a hospital or quarantine tank!!!