single entry evisa for VN is USD25. Cambodia evisa in advance is more expensive than Vietnam. Not sure what Cambodia's fee is on arrival but I bet it's not cheaper than USD 25
Sihanoukville is a shithole but it's where you need to go to get to Koh Rong, which is stunning. Kep is boring and the beach is crap, Kampot is a much better choice despite it being a riverside town and not coastal.
If she arrives on, for example, 1 April 2025 then she can stay for 3 months from that date.
I can't remember what subclass visa I had for Australia but mine expired on 3 Oct 2024. Exactly same conditions re 3 months per entry. I landed on 30 Sep and left yesterday, 5 Oct. No problems entering or leaving.