Unless there was a visible leak in the tourqe converter (which still requires you to drop the inspection cover), there is no way I can think of how you find a bad converter over a bad trans. Seems fishy to me.
But if they had to pull the trans to do that work, then they would have been on both sides of the motor and could have caused the engine oil leak. Even if the trans work was done perfectly, if someone failed to do the oil change correctly with something simple like making sure they didnt end up with a double gasket on the filter replacement. So let us know what you find out!