I highlighted my daughters hair and 2 weeks later she went swimming and her hair turned green how can we fix it

That's funny! Happened to me many years ago with my blonde hair and a pool. I just waited for it to grow out and wash out, which I think did wash out. Does she dislike it? Her friends might think it's cool. Better than overtreating her hair.

You need to go to a professional they will use purple to counter act the green to get your hair back to normal that's the only way