it depends on several things.
what species are the clowns/ other tank mates? is he planning a semi aggressive tank?
if he wants a semi aggressive tank, then I would say a yellow coris wrasse, a dwarf angel (find one taht wont nip but be ready to remove it if it does) or maybe some more peaceful damsels, like yellow tails.
if he wants a peaceful tank, then flasher and fairy wrasses, firefish, watchmen gobies, small gobies like neons, and chromis are all good fish. just make sure that you have either 1 or more then 3 chromis.