They can sue if they can prove that the code wasn’t reversed engineered in a clean room. Meaning nobody looked at the original code. And one person or group examines the system and writes the specifications and another group implements the specification without the teams interacting with each other. And usually a lawyer has to be involved and review the specification. The separation of teams is called the “Chinese Wall”

And depending on interpretation of the law using a decompiler can be seen as breaching the “Chinese Wall” since the implementation is then not based solely on the specification but based on the original code.