The encounter at DUB will have been documented and this is simply destined to repeat itself. Faced with an individual who has little or know ties back to Italy, a SO in the USA and who says he's headed to OZ, but doesn't have a ticket or visa for that, OP's SO will simply be denied entry again.

If the carrier is more careful this time, he won't even be issued BP's.

The SO needs to get his visa for the US and OZ and then buy a ticket Italy-USA-OZ-? OP can then buy the same flights USA-OZ-?

Since OZ won't admit either of them without onward travel either, the "?" needs to be resolved as well.