Hi, Richard. Sorry for the delayed reply here.
In this situation, I typically recommend that you just go to the site like they could and enter all the information for them: their name, email address, additional information, etc.
This is the only way to make sure your inventory numbers and additional information fields are fully completed as expected.
You may need to temporarily enable a different payment method or just enter their payment details in the WooCommerce checkout form for them (check with your PCI compliance officer about this first though).
Please let me know if you have any follow-up questions on this topic.