If starting over with this 20G tank and new fish, a few suggestions for your friend to consider are Tetras, notably Neon or Cardinal Tetras which are very popular in the hobby & American Cichlids including German Blue Rams and Bolivian Rams these get the cute\wow vote from me.
I guess your friend needs to see what fish they like the look of and then check out requirements of those fish notably size of tank and potential for tank mates.
You mention your friend is a beginner? I assume your friend is aware of
nitrogen cycle and that water should be tested and changed partially on a regular basis? it is good they have your to help.
Good luck to your friend choosing new fish.