The partner of a thief hates his own life;
he hears the curse, but discloses nothing.
If you’ve seen the TV show Law and Order, you have observed people aiding and abetting criminals by refusing to testify or actually perjuring themselves from the witness stand. Those people stand condemned if caught in their wrong-doing (TV never calls this a sin). The context of this proverb has to do with appearing before a judge during a trial. Leviticus 5:1 says, "If you are called to testify about something you have seen or that you know about, it is sinful to refuse to testify, and you will be punished for your sin" (NLT). A witness who refused to testify was seen as a partner-in-crime and would receive the same punishment (or curse) as the evil-doer.
James the apostle wrote, “Whoever knows the right thing to do and fails to do it, for him it is sin" (4:17). When we have the opportunity to do good, we should do it. It is right and pleasing to God.