你呢 translates to "how about you" and is usually followed by question mark not exclamation mark. 呢 is used as a way to express question.
Google translate sometimes does funny things. Cross check with other sources before trusting it.
If someone says 周末愉快! to you, reply to them with 周末愉快! is a safe bet. Or you can say Thank You.
If the example conversation actually happened, it could be that when A says 周末愉快 and B interprets it as a question for how was/will be for your weekends. So B response with a nod and asking A about his/hers. Though even under this circumstance it still seems pretty strange. We don't usually speak in this manner.