I'd definitely recommend the Great Ocean Walk to a international visitor. It is beautiful, lots of wildlife (especially koalas), navigation is easy, the ocean means it is pretty mild even on a hot day and your fire plan is easy, public transport accessible at both ends (saves having to leave a rental car in the bush for 7 days), and the 12 Apostles at the end is a real talking point. It is not true wilderness - facilties equivalent to what you would find on the Bibbulmun (ie. water tanks, composting toilets and open sided shelter).
Other PT accessible walks would be the Great Divide/Goldfields Track between Bendigo and Ballarat.
http://gdt.org.au/ There is also a bus 3 days/week to Mt Hotham in the alps.
A few things you should really impress on him about local conditions:
* wildfire is a major risk in Jan/Feb, and a local risk in Dec and March. Keep an eye on the weather forecast (hot days with a north wind are dangerous), and have a fire plan.
* water points are likely a lot further apart than he is used to - carry 2 litres at a minimum in summer
* being in a different hemisphere screws with your sense of direction - the sun is in a different part of the sky, moss is on the south side of trees, etc. Take extra care with nav.
* know the treament for snakebite
http://www.avru.org/firstaid/firstaid_snake.html Australian snakes are from a different class than most venomous snakes in the US, and the toxins work quite differently.
(I don't mean to say he is an idiot - just that some of these conditions are different from what he may be used to).