What should someone who works at a tech company that isn't associated with the work list as affiliation? I am preparing to submit a paper with a friend who is working at a tech company.  The company that he is working with is in no way associated with our research work.  Is it appropriate to list his employer as his affiliated institution?  Otherwise, I would leave his affiliated institution field blank.  Or, should I list the institution where he graduated from 2 years ago?

Update: Thank you very much for all of your feedback, answers, comments, and suggestions.  We really appreciate it.  I discussed with my collaborator and we will likely be listing him as an "Independent Researcher".  Although, we concluded that it wouldn't be unreasonable to list his employer purely as a personal affiliation (or point of contact) as long as he got approval from their HR department.
The co-author of my recently accepted paper works at Microsoft. The paper has absolutely nothing to do with her job, so we specified her affiliation as "Independent Researcher".