I assume he's an Australian with both a British and Australian Passport?
Things get hairy in early 2019
https://www.gov.uk/guidance/advice-f...ving-in-europe
However if he is an Australian Citizen with a British Passport then things aren't so straightforward.
Some EU countries will consider the Australian Citizenship (or other citizenships) of British Passport holders when determining whether residency can be permitted, even if that person was born in the UK.
If he's born in Australia, has claimed a UK issued passport by descent then most EU options are off the table following 29th March 2019. The withdrawal agreement does not cover Australian citizens or those of other Commonwealth countries who hold another citizenship in addition to British citizenship.
If he's born in the UK and is an Australian Citizen then many countries won't cover him under the withdrawal agreement unless he relinquishes Australian Citizenship.
If he's born in the UK and is an Australian Resident with permanent residency then he may jeopardise his permanent residency by becoming a resident of certain EU countries.
Before taking any decision on moving to the EU while Brexit is going on he should get clear written advice from the UK Home Office, DFAT and the Embassy of the country he is thinking of moving to.