Surely you have a authoring convention?
In my field, it is well established. It's very rare to find single author papers, as at the very least you will be working in a research group, even if a particular project is your own work.
Primary researcher (you) gets first author. First author is very prestigious. Last author is the head of the research group, or the person who has overseen the project. In between is anyone else who may have contributed, in order of how much work they did.
That is rigidly respected, and nobody who is second or third author would ever get more credit than the primary.
So if you were first author, your manager last, it's clear you've done the work, your manager has managed. If you put colleague second, she'd be "second author"'so everyone would think she has had something to do with the project, but would still know the work was yours.
Isn't it the same in all fields? I don't get how someone could not be first author, then down the line claim they were, certainly no one would invite a second author to conferences over a first.