B engine oil leak

[glenn in camino]

I recently overhauled an old Model B engine, got it running on my engine stand, and sold it to another Model A guy. He put it in a car and ran it for a few miles, after which he called me and said that the engine was leaking oil. He took it apart and found that the rear main is coming apart. It was ok when I put it together.
I don't know what to do. I'm thinking about asking him to bring it back so I can have the block rebabbited and possibly have the crank turned. What do you guys think? I still have most of his money.