QBO Australia to USA

Yes, he can still access the company file and use his subscription from Australia to the USA, @LynnT.

Since he's from another state, activating Multicurrency is necessary for accessing the company file.

Here's How: 

1. Head to the Gear icon, then select Account and Settings.
2. Select Advanced.
3. In the Currency section, select Edit.
4. From the Home Currency ▼ dropdown, select your currency.
5. Choose the Multicurrency checkbox and confirm that you understand that you can't undo this option.
6. Click on Save.

They can also sign up for a new subscription if they want for the US-specific features.

They can also export the Reports and Lists as individual Excel files in a zip.

To do that:

1. Access the Export data option by clicking on the Gear Icon.
2. Choose Export data.
3.  Select the date range on the Reports tab. 
4. Add or remove items from the Reports and Lists tabs by toggling the switch. 
5. Finally, click on Export to Excel.

Once done, they can import products and services into QuickBooks Online (QBO) US account. To

import your list ,follow the steps mentioned in step 2 of this article.

You can check this out for future reference on how you can customize your sales forms in QuickBooks Online.

If you have more concerns related to the Company file in QBO, don't hesitate to reply, I'm always here to help. Have a great day.

@LynnT 

You can purchase a 3rd party service to convert data from QB Online AU version to QB Online US version.