I'm glad to assist you in smoothly transitioning your client's data from QuickBooks Desktop to QuickBooks Online, @mwendel.

Certainly! You can import your client's file from the thumb drive into your new QuickBooks Online (QBO). Before doing so, you'll have to open the exported file from your thumb drive in QuickBooks Desktop (QBDT) to make sure it's formatted accordingly. 

Afterward, proceed to migrate and import it into QBO. If you do not have access to the QBDT program, you can download and utilize the 2021 trial version to complete the process.

When you're ready to move a file from QBDT (Pro or Premier) to QBO, follow these steps:

1. Sign in as an admin to the company file you need to import.
2. Go to the Company menu, then select Export Company File to QuickBooks Online.
3. Click Start your export.
4. Sign in as an admin to your QBO company.
5. Select the Choose online company drop-down, and select the QBO company you want to replace your company file.
6. Click Continue.
7. Enter Agree in the text field, then select Replace.
8. Select Yes, go ahead and replace the data, then click Replace.

You can also read this article for the complete details: Move your QuickBooks Desktop file to QuickBooks Online.

For additional reference, you can use this article as a reference to understand how features and data move from QDT to QBO: Learn how features and data move from QuickBooks Desktop to QuickBooks Online.

Feel free to update me on your progress or ask any questions in the comments. I'm here to assist you every step of the way. Good luck with the migration!