I'm from Perth, though currently living in Las Vegas, I go back usually a couple times a year. I have also lived and worked in many different areas of the state of Western Australia as well as several other cities in USA and Europe. Perth is still my favorite. It is a cleaner, friendlier, cheaper and safer version of San Diego. So few visitors to Australia see it though as they stick to Sydney with a quick trip to Queensland most of the time.
If she is staying out of the city or even on the outskirts, she will need a car to get around. For a three month trip, she could look for a good monthly rate from a place like
http://www.bayswatercarrental.com.au/ (maybe ~US$2500 + mileage for a small car) or buy a car for $4-5000 and hope it lasts as was suggested earlier, but then you gotta sell it at the end.
There is a lot to do and see in and around Perth and of course elsewhere in
http://www.westernaustralia.com/en/ . It all depends on what time she will have off. It is "cold" and rainy for a lot of the Winter unless you go North. There are plenty of weekend destinations 1-5 hours away. If she has a week or more off she can head up the coast.
Compared to the US, gas is expensive as are domestic airfares.
I can guarantee you she will love it there, everybody does
