What's she mean by "look at you?" Do people commonly say this?

Native speaker’s answer
Rebecca
Yes, people do commonly say, "Look at you!" It's usually said when expressing happiness or surprise by what someone is doing or how they look. It's similar to saying "wow!" In this context, she's saying, "look at you," since they haven't seen each other for a while, and she's happy to see him. It's also to comment on how much he has or hasn't changed physically in the time since they've seen each other. Ex: Wow, look at you! You're so good at painting. => watching someone paint. Ex: Look at you! You look so great. Have fun at prom! => A sort of compliment on how someone looks Ex: It's been such a long time. Look at you! You haven't changed a bit.