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.