You shouldn't sleep over at a significant other's friend's house.
Sarah's husband Jay is away on a business trip, and she needs someone to help her study her flashcards for an upcoming nurses exam.
Sarah wants Jay's friend, Tom, to help her study.
Sarah asks Tom if he can come over the next morning to help her study.
Sarah goes over to Tom's late in the evening to study, when he suggests that she stay over.
Sarah and Tom study over breakfast before Tom has to leave for work.
One thing leads to another and Sarah and Tom end up sleeping together.