In a tank that small, the Cory cats would just stress the betta.
Check out the pin in the fresh and planted nano section. There are great ideas there.
For instance, you could get some species of killifish, a shrimp tank, snail tank, betta tank, some gourami, dwarf puffers (very tiny, big personality), I think a few tetras are on the list.
Of course your friend will need to cycle the tank and be prepared to be liberal with water changes as small tanks experience issues much more heavily compared to a larger tank which can better handle diseases, changes in water conditions, and other calamities.