Google translates "xin lỗi, bạn đã bị cho leo cây" as "Sorry, you were climbing trees"
It's slang lesson so...
Is this an expression of sympathy or an apology for doing the standing up?
Probably the former: to sympethize. If it was an apology, it would be "Xin lỗi vì (tôi) đã cho bạn leo cây"
Can someone elaborate on what this means?
This means, these two people are having an appointment, and the second person, "you", was left waiting for a very long time, and maybe they don't even see the other person come.
Maybe the waiter says this to the poor person waiting alone at their table for hours after their date was supposed to arrive.