I am developing an add-in for desktop Excel. I want to send it to the client so they can test it. We're not on the same network, so the article: Sideload Office Add-ins for testing from a network share doesn't apply.
Does the manifest.xml have to be on a network share, or can they just load it directly?
I tried following the official documentation but it doesn't cover my case.
First of all, you need to make sure the web application is hosted on the web server which is accessible over the internet, so you could send the manifest file only. That will be enough, see Manually sideload an add-in to Office on the web for more information. There is no requirement to host the manifest file on the network share, just a convenient way of sideloading add-ins.
See Test Office Add-ins for more information.