What should my friend do who works for a private company and suspects that the owner's money is ill-gotten?

It is not permissible to work for a company that deals in unlawful things, such as selling alcohol or usury, because Allah Almighty says: "And cooperate in righteousness and piety, but do not cooperate in sin and aggression." For he who assists in disobedience is like the one who commits it.

However, if there is nothing in the company that violates Islamic law, then there is no harm in working for it.

And if the owner's wealth is doubtful, then it is more cautious to leave working with him and seek another permissible job that is free from doubt.