Can an author refer to work they did jointly using the first person? 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?
In maths you never use the first person singular anyway, so they question as stated does not apply. Also, all work in a paper (in pure maths at least) is understood to be the work of all authors. In fact, it is common to refer to the other paper in the third person, even when the authors are the same.

In short: no, this is not acceptable.