Puddles on passenger floorboard?

My wife has a 2000 Ford Taurus. Months ago we got some work done to the heater core. Now the passenger side of the car is leaking and theirs puddles that show up on the floor. I'm guessing it has something to do with the heater core. What could the macanic have done wrong in order for this to happen. I'm just looking to be well informed before I go back to where the work was done and tell them they screwed up…

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