@TheDog Thanks -- it's not Defender that's the issue, it's Windows 11's new "Smart App Control", which is an additional defense mechanism that has no "run anyway" feature -- it just triggers when Windows doesn't recognize an app's files as being 'registered' and safe. It can automatically flag older or hobbyist-made independent apps (like TripleA) as malware and refuse to run them, unless to permanently turn on "SAC". Frustratingly, It won't give you any specifics as to why the file is on Microsoft's "naughty list".
My friend just contacted me and told me he's now getting an "OK" message, and Windows will run it now, so...problem solved?