Thanks for providing us the screenshot. Let's perform some steps to let your customer access the sent invoices, nathansmith.
There are times when a browser's cache data becomes full or damaged. When this happens, pages don't load properly.
Please ask your customer to use the browser's private browsing mode to access the sent invoice. This mode doesn't use the existing cache data and helps us confirm browser-related issues.
If it works, they can go back to the original browser and clear the browser's cache to delete the temporary Internet files. They can also use a supported, up-to-date browser.
Keep me posted if you need anything else with sending invoices to your customers. Just reply to this post and I'll be here to answer them.