
I would have said buy a piece of 18mm WBP that is bigger than the hole (4'x2' perhaps), lay it over the hole, mark the floor, whiz a circ saw along the marks, pull out, slot in, bit of glue and filler, bob's yer uncle.
If you are sure the polystyrene is not damaged you haven't got to take it out. the board will bridge over small defects.
She doen't want to take up the whole floor.