The first coworker who comes to mind for me is a guy I worked with whose sister and brother-in-law died in a car accident. As next of kin, he took the kids and eventually adopted them, but as far as I'm aware, at no point did he ever call himself their parent and I think that would have seemed oddly distasteful to do so. Uncle still works even when you're raising them.
Yep I can definitely see that, I could see myself making the same decision in that situation. I would probably reach for uncle, caretaker, or guardian. I'm still lost as to when "child-raiser" would ever be needed though, and personally feel like avoiding saying parent would be more offensive to more people than the other way around.
I.e. I could see your coworker correcting someone and saying they aren't the father or parent with little or no offense taken. Though I could see referring to someone with adopted children as "child-raiser" as being really offensive to the person, as though I effectively took away the term "parent" and everything that goes along with it.
