Ugh, nasty, especially when you mix other clients into the data, I would suggest this could only be fixed by a custom script, charge them a fortune as a lesson and hope like hell you can deliver. I would then implement a logging/audit strategy on the database or separate each client to their own environment. If you screw up another clients data b/c of this issue your whole business case just went out the window!
Never underestimate the power of human stupidity RAH
