Friends are looking at buying a house. Nice old 1890ish terrace, two
floors and a full-size cellar.

Unfortunately some idiot has previously removed the chimney breast from
the cellar level, leaving the chimney above unsupported! They've
removed the hearth slab and mantel, so the weight isn't too bad, and
they've filled the gap with matching floorboards (stripped floors).
However there's a 4' x 1' chimney breast hanging there with nowt
beneath. If you look up from below, you can see right up the flue and
practically out of the top.

Any suggestions on fixes?