Aussie is only getting the i9505 snapdragon powered LTE model. Try asking him to go to a Harvey Norman and see if he can pre-order one and ask if no stock whether can cancel and refund. HN is taking pre-orders for $819 with free sandisk class 4 32gb micro sd.
Carriers and shops like Harvey Norman, JB Hifi and DickSmith are most likely to receive stock of the i9505 S4 by this Saturday or within next week, though heavy pre-orders mean not much stock to go around. There are Samsung stores in Melbourne and Sydney that guarantees to having stock this saturday and taking pre-orders but by now, first stock might have become exhausted and Samsung stores sell at full $899 retail price. Note that most places would probably allocate more stock to prioritise people going on contracts. You could try asking the Telstra or Optus shops to sell one outright though whether they'll sell it to your friend or not depends on luck some times. Telstra and Optus usually do not lock non-prepaid phones, vodafone provides unlock codes for majority of models for free on their website by just entering the phone's IMEI number. Aussie carrier branded phones are heavily bloated with carrier bloatware and carrier splash screens when starting the phone so if you do get a carrier branded one, you might want to flash a different ROM.

If he did somehow able to secure an aussie stock phone, make sure he takes advantage of the Tourist Refund Scheme and claim back money for GST at the airport
http://www.customs.gov.au/site/page4646.asp