How should a university non-academic employee's affiliation be like? A friend is in a situation where they obtained a PhD in one field in a different country, but now is working at a university in an unrelated field in a non-faculty, non-research position. However, they have got invited to a conference to present on matters relating to their PhD field. How should they list their affiliation? They use their current university's library facilities, etc, for their own private after-hours research, so they want to list their current university employer, but I feel that this is unethical, and "Independent Researcher" or similar would be better.

I've seen a number of similar questions here relating to how students or researchers should handle this, but I think this situation is sufficiently different to merit a new question.
I have faced this problem in a previous publication, and what I did was list my current employer as my affiliation, with an asterisk, stating that "Large part of this research work was carried out while I was with the ..."