"Rick-Meister" wrote

You really are playing with fire here. Once wood rot starts, no amount
of filler is going to put it back into shape. There are several ways
to jury-rig a temporary patch by cutting out a large section of
sub-flooring from underneath and then installing plywood patch pieces
and braces. But NONE of these fixes are long term. The only way to fix
this properly is to remove the tile and install new sub flooring.

If you do this temporary patch and try to sell your house down the
road, you're going to get nailed big time by the inspector.

Agreed. First thing is to find the leak that caused the rotting. He may
have already fixed that but not mentioned it?

He may have serious structural damage that is ongoing and patching it for a
few weeks til he can get it fixed might be 'ok' but thats just a few weeks.