I thought it was called "parent child," which I remember from a conversation with a friend in therapy, but now I can't find any indication of such a condition on the internet.
It's just mentioned in passing in my story (set in the US, a couple years ago) so I don't need much detail on the condition itself. I'd mostly like to confirm that there is such a term and whether I have it right or wrong. Or if someone knows of a different term describing a similar thing. ETA: The term is "parentified child." Would such a term be found in a psychology text book?