A management system my photographer friend can use without learning what "CLI" or "GitHub" are. They just login and make changes.

Yup, exactly that. WordPress editors (WYSIWYG and Gutenberg) are still far more user-friendly and mature than a Netlify CMS that still is buggy like hell and where many little details and YAML formattings need to be fixed in code editor. You really cannot expect such a thing to be done by a non-coding person.

There's https://www.netlifycms.org/ if you want a static site generator with a CMS, FYI.

Are you aware of any open source applications as a CMC for static generators like Jekyll? Would be very convenient to have one with a a simple github login/auth settings page and then add/edit page/post options and the ability to select or upload a theme would be great.

NetlifyCMS works with Jekyll, Hugo, Gatsby, Middleman and a few others.

https://www.netlifycms.org/docs/jekyll/

my photographer friend can use ftp

my photographer friend can also add <img> tags to static html documents.

A huge variety of plugins, and a large pool of cheap developers with plenty of experience in the stack. I personally like Jenkins, but if I’m handing the site over to a low-tech client, I’d use WordPress hosted on managed WordPress.