A friend has downloaded my source code from the repo and, while trying to set up his environment, had 3 errors on his first build:
Processor ‘FontDescriptionProcessor’ had unexpected failure! twice and "The command dotnet mgcb failed with code 2. I assume this is an environmental problem.
Can anybody offer any suggestions that I can forward to him?
Sounds like the ttf fonts you are using are missing on his system.
Export the font from your windows directory, happy hunting 
Though it has improved in recent years…
Ensure you package all external assets with your distributions, and that you have permissions to do so…
I just checked and the TTF fonts are in the repo CONTENT folder. Do they have to be installed on his machine, too?
Try the root exe folder
To refer to a specific font file (TTF) instead of a machine-installed font, your .spritefont must refer to it via the filename/path, instead of the font name. I.e. place your FontName.ttf file next to your SpriteFont.spritefont file, and within the SpriteFont.spritefont xml, refer to the font using <FontName>FontName.ttf</FontName> (as opposed to something like <FontName>Font Name</FontName>)
Thanks. I’ve passed this link on to my friend.