[favorited]

It's also likely a copyright violation to use that font file without permission.

Typefaces have never been copyrightable in the US, but digital fonts implementing a typeface have been since the early 90s.

[seandougall]

This all sounds like it was done without duplicating the font itself (aside from installing it, which does make a copy in one of the Library/Fonts folders, but afaik that isn't been considered duplication for copyright purposes). And it could be done easily enough without even making that copy (a symlink would likely do it).