The suggestion given in the Harvard admissions blog states: "Of course, we certainly understand that some applicants have been out of school for several years or might otherwise be unable to secure a letter from an academic source. In such instances, we suggest asking your professional reference writers to comment on qualities and skills that can translate to an academic context."

So just pick someone who can speak to that and ask them specifically to touch on how the qualities and skills they witnessed in a professional environment would translate to academic success.