What should someone who works at a tech company that isn't associated with the work list as affiliation?

Question

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.

Answer 1

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".

Answer 2

Speaking as a scientist who works and publishes frequently from my position at a tech company: his affiliated institution is his company, and it is entirely normal and appropriate to list it as such. It would be incorrect to list his affiliated institution as his alma mater, since he is no longer employed by them.

Note: My answer assumes there is no objection from the company to being listed. If there is, then the answer from Sergey Dymchenko applies.

Answer 3

On the legal side, the answer to this question is linked to an issue that I do not see mentioned in any of the answers yet. It depends on your friend's contract (and possibly your local legislation)

• to what extent your friend's employer allows your friend to perform other "work-like" public activities outside of their paid working time (often, only directly competing activities are forbidden, and from the question I understand that the company is not related to the research activity, which implies that the research was entirely done in your friend's private spare time), and
• to what extent your friend's employer is legally allowed to indirectly take ownership of something they did not contribute to.

On the ethical side, the answer hinges on the intended purpose of listing the affiliation:

• Does the affiliation say "This organization made the research possible."? In that case, listing an employer who had nothing to do with the research (and just happened to be your friend's employer at the time the research was done) would be deeply unethical, comparable to adding an author who did not contribute to the paper. (Note that especially in this case, the solution of asking the employer for their preference might be counterproductive, as from a business perspective, the employer will not care about research ethics and go for the opportunity of placing an "ad for free".)
• Does the affiliation say "This organization might be interested in building upon the presented work."? In this case, listing your friend's current employer might seem entirely appropriate if they might become associated with (future parts) of your research work (even though they didn't have any part in the current paper).
• Does the affiliation say "This is where you, future reader, can contact author X."? In this case, the most appropriate "affiliation" might be either a personal affiliation (e.g. a personal website), or indeed the last affiliation that was actually involved in the research. We routinely do this, for example, for students who support us in writing a paper that involves topics from their graduation thesis (and thereby become co-authors) - who, due to their graduation, have already started working at some unrelated company at the time of writing the paper.