According to Jim - Season 3 Episode 4 Getting to Know You

Cheryl says a friend of hers got jewelry for his wife for no reason and Jim says he's probably cheating on her. After Cheryl takes it offensively that Jim won't buy her jewelry because he doesn't think he has to – after all, they are married and he isn't cheating – he goes out to shop for earrings, only to find out he knows nothing about Cheryl. After spending three days dedicating himself entirely to paying attention to Cheryl – with a bit of Andy's help – Jim finds out that Cheryl's friend was indeed cheating and they finally make up.