I believe that a RTW ticket requires a minimum of three "stopovers". I also think that current RTW Star1 fares ex-USA are about US $4,500 for Y, but travel to OZ may entail more miles that Star1 permits (and therefore, more $$$.) Economy RTW tickets book into M class, which is fairly high on the pecking list, so more likely to be available when she wants versus the really low fare buckets.

I recently priced a B fare LAX-MEL that came in around $3,500. It allowed changes, but carried a cancellation fee of $100 but only if cancelled prior to travel. It was non-refundable once travel began. A business class R/T D fare was an astounding $10,400!

She/you might look into purchasing two one ways tickets, unless she wants to decide to return at the last minute.