Translation:My friend is Australian.
Those people posting about "My friend is an Australian", well there is reason to be wrong. When using "is an australian-) you are using "Australian as an adjective, no? In contrast to " My friend is Australian", where the australian is used as a noun. "My friend is an Australian student/teacher" would be the proper way to use "is an australian" . I hope this post helps out other people, and have a great day.
When you use plural, the Chinese change 朋友to 朋友们. But I think it should be acceptable.
It is, arguably, incorrect to say in plural because Australian is an adjective. Otherwise we would have said Hugh Jackman is an Australian rather than Hugh Jackman is Australian.
朋友們 should not be used in the Chinese sentence. Remember there is no plural in Chinese. 們 is only affixed when emphasis on sense of unity is needed.
e.g. (To a crowd) 朋友們，不要放棄 / Fellows, don't give up.
I'm just suggesting an alternative if he desperately want to express a plural form.