It might not be that simple. Assuming the person is going to be paid in USD, then unless they have a large pot of savings (or some other income) in Australia, they will be hit twice. Every transaction will be converted into AUD by Citi and then every month there is a bill to pay, which means cash will have to be transferred from a US bank account in USD, to an Aussie bank account in AUD. The initial credit card transaction will definitely incur a fee (3.3%). The transfers from the US bank account to the AU bank account will probably incur fees, or alternatively the banks will make money by using very unfavourable exchange rates (effectively the same thing as a fee). In some cases banks will do both, meaning you really get shafted, and this is especially the case if you’re transferring relatively small amounts (you can get much better rates through fx brokers if you transfer large amounts, e.g. $20k+, but highly unlikely you would be doing that each month to pay your credit card bill).
There is also a risk of losing out due to exchange rate fluctuations between the time when you make a purchase and when you pay the bill. This is a particular issue if you make a large purchase near the start of the billing cycle. For example, spend USD$5k today in the US, and approx AUD$4850 will be added to your bill. But if the AUD suddenly strengthens further vs the USD, say back to AUD$1 = USD$1.10, then it will cost you USD$5,320 to pay off the purchase you made a few weeks ago for USD$5k. This could also work in reverse of course, meaning you could gain, but basically you’re just blindly gambling on fx rates every time you use your card. This is not really an issue if your income is in the same currency as your credit card account.
You also need to factor in the AUD$700 fee for the Citi Select card in the first place (assuming the person in question doesn’t already have one, which is the impression I get). Also, it’s possible to get credit cards with decent rewards and benefits in the US for much lower fees than here.
Generally speaking, if you’re moving to a new country for a significant period of time, it’s safer and a lot less stressful to bank solely in the currency of that country while you’re there. Obviously it’s different if you’re just going on holiday or a short stint working abroad.