Another way of saying this is 'to shove the boots in' meaning to take advantage of someone when they are already poor, unhappy, depressed etc.

It's not a common figure of speech, but given the imagery I'd say it means: "put more problems onto somebody who already has a lot of problems", or maybe "make a problem worse" e.g. John was having a terrible week - he lost his wallet on Monday, and he got the flu on Tuesday. Then, on Wednesday, his fiancee tossed another shovelful of dirt on John's coffin by breaking off their engagement. or: Jane's car was terrible; it was 27 years old, it broke down every month, and it used lots of petrol. Yesterday, a truck driver tossed more dirt on her coffin by driving into her back-end when she was out shopping.

How about this as an example: you use deception to rob a man of all his wealth - then you take his wife as well.

another saying is "add insult to the injury".

another saying is to 'rub salt in the wounds'