I need a clear answer. Did I commit sin if I have known from the beginning my friend has committed adultery and still dating someone? She still married, still living in the same house together with her husband and kids. Her husband is nice, old, and busy at work all the time. I also live in the house. As my friend she tells me about her guys, but her husband knows nothing. Am I committing sin? What should I do? I am just a listener. If the husband and kids are out of town, she take the guy in the house and sleep with him. I know it. She doesn't hide from me. Please give me an answer!
Your friend is committing adultery and you are doing her no favor by not telling her she is wrong. "When I say to the wicked, 'O wicked man, you shall surely die!' and you do not speak to warn the wicked from his way, that wicked man shall die in his iniquity; but his blood I will require at your hand. Nevertheless if you warn the wicked to turn from his way, and he does not turn from his way, he shall die in his iniquity; but you have delivered your soul" (Ezekiel 33:8-9). It is she who needs the warning that her eternal life is in jeopardy. "Marriage is honorable among all, and the bed undefiled; but fornicators and adulterers God will judge" (Hebrews 13:4).
Whether the husband is fooled or suspects but doesn't want to know, I cannot tell. But the obligation to warn is to the one sinning, not necessarily to the one being sinned against. Your salvation is connected to your willingness to stand for righteousness.