I'm not so sure that it is the right idiom for this situation. "It never rains but it pours" is used to imply that a number of bad (or sometimes good) things happen one after the other in fairly quick succession. A similar British English expression is "it's one thing after another". I would imagine that some time would pass between a fatal accident and the subsequent court case - there would have been an inquiry into it for a start. Is that really a quick succession of bad events?
I would think a better idiom would be "to add insult to injury", which means to make a bad situation worse. For example:
Not only had the accident cost his son's life, but, to add insult to injury, his counsel failed to conduct his case properly in court.