I would try Neon Tetras perhaps 7-10 of them.

Yes, Neon or Cardinal Tetras would work nicely, or perhaps a few Zebra Danios.

Either way, your friend will need a heater for these fish, along with the usual filter, etc.

More unusual although brackish...

bumblebee gobies. They're not as zippy as the other guys but cute as heck ;)

Adrea

One male betta would be nice with a 2 cory catfish. Make sure the small aquarium comes with filter. The rule is for every 1" fish for 1 gallon of water. So the fish need to be small and few. If you have plants and gravel in the water that eliminates even more space of water and fewer fish.
Theresa