What should I do about my knowledge that my friend is committing adultery with his brother's wife?

The adulterer should be advised in the best manner, and the heinousness of his act should be explained to him. If he persists, he should be threatened that his brother will be informed of what he is doing. If this does not work, his brother should be advised with a general warning, without a direct accusation of adultery, due to the absence of four witnesses. Otherwise, the accuser would be subject to the prescribed punishment (hadd).