Wow, your screwed. I'm at a loss for once. I suppose I would build up a
wall, sitting on the new floor, up maybe 16" and out maybe 6" reinforce
with rebar and anchored to the exiting wall and floor. It wouldn't
really be structural but would certainly keep the dirt from under the
foundation from pouring out.

Why didn't the guy stop when he realized he was undermining your
foundation?
Why didn't you stop him before he got too far?
Just curious.