what can be done

I am writing this on the behalf of my one of the male friends. He's a good buddy of mine and shares almost everything to me. Recently he told me something. He's been living with his brother and sister-in-law for the last one year. His brother has a job. He is studying. He and his sister-in-law are often left alone in the house. For the last couple of days, they developed some feelings for each other. She shared everything with him and even told that her sex life is terrible. Eventually, they hugged and kissed each other. But my friend soon took his steps back otherwise situation would have turned into sex. Now, the problem is he is having strong guilt of why it happened. Also, his sister-in-law has developed very strong feelings for him after kissing and stuff( But no sex happened). He asked her to not let anything happen in future again. But she is not ready to leave him. She likes him now. She even emotionally blackmails him to not to leave her. He wants to build his career but with this guilt he won't. He is confused and guilty. What should he do?