Water damaged subfloor in living room
It would seem to me that someone could clean out the old rotted stuff, piece in a new sub-floor piece. Under that put another piece of subfloor blocking it up between the joist. As for who can/should do it...ask your refinisher, or check the local papers for a handyman of sorts. It wouldn't be that difficult, and I would at all cost avoid messing with the hardwood from the top side, considering that the subfloor is open from the bottom.
when it comes to breakfast, the chicken is dedicated, the pig is committed.