I wrote a paper with a collaborator. The collaborator wrote a followup paper where they referred to their work in that paper in the first person, i.e. "in a previous paper, I presented the idea that...". To be clear, that particular idea really was their idea, not mine.
Is this acceptable, or should they have instead said "we presented the idea that..."? Does it differ between different fields, such as computer science, mathematics, physics, and economics?