What is the ruling on remaining silent regarding the cheating of two colleagues against another colleague at work and usurping his rights, and is the silent one considered a partner in sin?

It is not permissible to remain silent about what the two partners are doing. The third partner must be informed of the truth, unless harm is feared from them. In that case, he can be informed through an intermediary. Your silence about this wrongdoing is not permissible, except in the event of fearing harm to yourself and your wealth.